Cache apparatus for enabling overlap of instruction fetch operations

ABSTRACT

A data processing system comprises a data processing unit coupled to a cache unit which couples to a main store. The cache unit includes a cache store organized into a plurality of levels, each for storing blocks of information in the form of data and instructions. The cache unit further includes control apparatus, an instruction buffer for storing instructions received from main store and a transit block buffer comprising a plurality of locations for storing read commands. The control apparatus includes a plurality of groups of bit storage elements corresponding to the number of transit buffer locations. Each group includes at least a pair of instruction fetch indicator elements which are operatively connected to control the writing of first and second blocks of instructions into the instruction buffer. Each time a read command specifying the fetching of instructions of either a first or second block is received from the processing unit, the flag storage element associated with the transit block buffer location into which the read command is loaded is set to a binary ONE state while the corresponding ones of the flag storage elements associated with the other locations storing outstanding read commands specifying instruction fetches are reset to binary ZEROS. This permits only those instructions received from main store in response to that read command to be loaded into a specified section of the instruction buffer for enabling overlaps in processing several commands specifying instruction fetch operations.

REFERENCED APPLICATIONS

1. "Cache Unit Information Replacement Apparatus" invented by Marion G. Porter, Robert W. Norman, Jr. and Charles P. Ryan, Ser. No. 968,048, filed on Dec. 11, 1978 and assigned to the same assignee as named herein.

2. "Instruction Buffer Apparatus of a Cache Unit" invented by Marion G. Porter and Robert W. Norman, Jr., Ser. No. 968,050, filed on Dec. 11, 1978 and assigned to the same assignee as named herein.

3. "A Cache Arrangement Utilizing A Split Cycle Mode of Operation" invented by Marion G. Porter, Robert W. Norman, Jr. and William A. Shelly, Ser. No. 968,312, filed on Dec. 11, 1978 and assigned to the same assignee as named herein.

4. "A Cache Arrangement for Performing Simultaneous Read/Write Operations" invented by Marion G. Porter, William A. Shelly and Robert W. Norman, Jr., Ser. No. 968,521, filed on Dec. 11, 1978 and assigned to the same assignee as named herein.

5. "A Cache Unit with Transit Block Buffer Apparatus" invented by Marion G. Porter, Charles P. Ryan and William A. Shelly, Ser. No. 968,522, filed on Dec. 11, 1978 and assigned to the same assignee as named herein.

6. "Command Queue Apparatus Included Within A Cache Unit For Facilitating Command Sequencing" invented by Marion G. Porter, Ser. No. 968,311, filed on Dec. 11, 1978 and assigned to the same assignee as named herein.

7. "Buffer Store Including Control Apparatus Which Facilitates the Concurrent Processing of a Plurality of Commands" invented by Charles P. Ryan, Ser. No. 853,982, filed on Nov. 22, 1977 and assigned to the same assignee as named herein.

8. "An Instruction Buffer Associated with a Cache Memory Unit" invented by John E. Wilhite, et al, Ser. No. 866,083, filed on Dec. 30, 1977 and assigned to the same assignee as named herein.

BACKGROUND OF THE INVENTION

1. Field of Use

This invention relates to data processing systems and more particularly to a data processing system which includes a cache unit for facilitating the processing of requests for instructions and operands.

2. Prior Art

As is well known in the art, many data processing systems each include a main store and a cache unit positioned between the system's data processing unit and main store for the purpose of increasing the performance of such data processing unit. Additionally, such high performance data processing units have included instruction buffers for providing fast access to instructions.

It has been recognized that even with a cache unit and instruction buffer, data processing unit's performance can be compromised by the transfer from one instruction sequence to a second instruction sequence. To overcome this, one prior art system includes an instruction buffer capable of storing two instruction sequences which can be available for use by the data processing unit. For further information regarding this system, reference may be made to the copending patent application "An Instruction Buffer Associated with a Cache Memory Unit", invented by John E. Wilhite, et al, bearing Ser. No. 866,083, filed on Dec. 30, 1977 and assigned to the same assignee as named herein.

While the above arrangement facilitates the transfer from one instruction sequence to another, it requires that execution of the command specifying the fetching of the instructions of one sequence be completed before the command for fetching instructions of a second sequence is issued to main store. During that time, the processing unit is required to halt its operation. The cache unit, upon loading all of the instructions of a block into the instruction buffer started the processing unit up in parallel with issuing the next command to fetch a second block of instructions.

Even in the case where it would be possible to issue the above sequence of commands for fetching first and second blocks of instructions in succession without interrupting the operation of the processing unit, it would still be necessary to halt operations until the execution of both commands had been completed before issuing another command for fetching instructions.

Accordingly, in either case, there would be a substantial decrease in the processing unit's performance.

Accordingly, it is a primary object of the present invention to provide an improved cache system.

It is a further object of the present invention to provide a cache arrangement which permits a high degree of overlap of commands specifying the fetching of instruction blocks from a main store.

It is still a further object of the present invention to provide a cache arrangement which facilitates the processing of commands generated by a processing unit in response to transfer or branch type instructions.

SUMMARY OF THE INVENTION

The above and other objects of the present invention are achieved in a preferred embodiment which includes a cache unit which couples between a main store and a data processing unit.

The cache unit includes a cache store organized into a plurality of levels, each for storing a number of blocks of information in the form of data and instructions. The cache unit further includes control apparatus, an instruction buffer for storing instructions received from main store and a transit block buffer comprising a plurality of locations for storing read type commands.

The control apparatus includes a plurality of groups of bit storage elements corresponding to the number of transit block buffer locations. In the preferred embodiment, each group includes a pair of instruction fetch flag indicator storage circuits which operatively connect to the instruction buffer for controlling the writing of blocks of instructions there. In greater detail, a different one of the pairs of instruction fetch flag indicator storage circuits is associated with the fetching of first and second groups, half blocks or blocks of instructions.

Normally, as a consequence of a branch or transfer, each time a read type command specifying the fetching of either a first or second block of instructions is received from the processing unit termed an instruction fetch 1 (IF1) and instruction fetch 2 (IF2), respectively, one of the instruction fetch flag indicators associated with the transit block buffer location into which the read type command is loaded is set to a binary ONE state. Corresponding ones of the instruction fetch flag indicators associated with the other locations storing IF1/IF2 commands previously transferred to main store, are reset to binary ZEROS.

Accordingly, the instruction buffer is conditioned by the states of the instruction fetch flag indicators to load therein only those instructions received in from main store in response to the last IF1/IF2 command. Therefore, there is no conflict with old blocks of instructions received from main store after a new IF1/IF2 command is issued since the instruction fetch flag indicators which enable the instructions of the old block to be written into the instruction buffer has been automatically cleared to ZEROS when the flag indicator was set in response to the new command. From there, the instructions are transferred to the processing unit.

In the preferred embodiment, the groups of bit storage elements include a cache write flag indicator associated with each transit block location which is normally set to a binary ONE and causes memory information to be written into cache store. In accordance with the binary ONE state of the cache write flag indicator, the instructions fetched in response to the new IF1/IF2 commands as well as the instructions of the old block, are written into cache store independently of the states of the instruction fetch flag indicators.

In accordance with the teachings of the present invention, when the transfer or branch instruction executed by the processing unit which produced the IF1/IF2 command is a no-go, the command is inhibited from altering the instruction fetch flag indicators. Also, the cache unit is able to issue a new IF1/IF2 command at any time without holding up operations of the processing unit until the execution of the previous IF1/IF2 command is completed.

Accordingly, the arrangement of the preferred embodiment of the present invention permits considerable overlap in the processing of IF1/IF2 commands without affecting the integrity of the instructions being written into the instruction buffer notwithstanding the arrival of instructions of old blocks in response to previous IF1/IF2 commands. Thus, the performance of the system is increased with respect to the processing of transfer or branch instructions for switching from one instruction stream or sequence to another.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying drawings. It is to be expressly understood, however, that each of the drawings is given for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in block form a system employing the principles of the present invention.

FIG. 2 shows in block diagram form the host processor 700 and the cache unit 750 of FIG. 1.

FIGS. 3a through 3e show in greater detail, certain ones of blocks of FIG. 2.

FIG. 4 shows in block diagram form the cache unit 750 of FIG. 2.

FIG. 5 shows in greater detail, the cache processor interface 604.

FIG. 6a illustrates the format of the control store control unit of FIG. 1.

FIG. 6b illustrates the format of the microinstruction words of the execution control store of FIGS. 2 and 3.

FIGS. 7a through 7e show in greater detail, different ones of the sections of cache unit 750.

DESCRIPTION OF THE PREFERRED EMBODIMENT General Description

As seen from FIG. 1, the system which incorporates the principles of the present invention includes at least one input/output processor (IOPP) 200, a system interface unit (SIU) 100, a high-speed multiplexer (HSMX) 300, a low-speed multiplexer (LXMX) 400, a host processor 700, a cache memory 750, at least one memory module corresponding to a local memory module 500, and at least one memory module corresponding to a memory module 800. Different ones of these modules connect to one of a number of ports of the system interface unit 100 through a plurality of lines of different types of interfaces 600 through 604. More specifically, the input/output processor 200, the cache memory 750, and the high-speed multiplexer 300 connect to ports G, E and A, respectively, while the low-speed multiplexer 400, local memory module 500, and main memory module 800 connect to ports J, LMO and RMO, respectively. The host processor 700 connects to the cache memory 750.

System Interfaces

Before describing in detail the processor 700 and cache unit 750, constructed in accordance with principles of the present invention, each of the interfaces 600 through 604 discussed previously will now be described.

The data interface 600 is one of the interfaces which provides for exchange of information between an active module and system interface unit 100. Exchange is accomplished by controlling the logical states of various signal lines in accordance with pre-established rules implemented through a sequence of signals termed a "dialog".

The interface 601 is a programmable interface which provides for transfer of command information from an active module and a designated module. The transfer is accomplished by controlling the logic of states of the various signal lines in accordance with pre-established rules implemented through a sequence of signals termed a "dialog".

A further interface is the interrupt interface 602 which provides for interrupt processing by the input/output processor 200. That is, the interface enables the transfer of interrupt information by an active module to the SIU 100 to the input/output processor 200 for processing. Similar to the other interfaces, the transfer of interrupt requests is accomplished by controlling the logical states of the various signal lines in accordance with pre-established rules implemented through a sequence of signals termed a "dialog".

A next set of interface lines utilized by certain ones of the modules of FIG. 1 corresponds to the local memory interface 603. This interface provides for exchanging information between local memory 500 and the modules of the system. The exchange is accomplished by controlling logical states of the various signal interface lines in accordance with pre-established rules implemented through a dialog sequence of signals.

Memory and programmable interface commands are transferred out of the same physical data lines of the interface. The interface does not include a set of lines for processing interrupt requests and therefore the modules connected to the local memory by the SIU 100 cannot directly cause a memory interrupt.

For a more detailed description of the elements of FIG. 1 and each of the interfaces 600 through 603, reference may be made to U.S. Pat. No. 4,006,466.

The last interface 604 is an internal interface between the cache unit 750 and central processor 700 which corresponds to the cache/CPU interface lines of FIG. 5. This interface provides for exchanging information and control signals between the processor 700 and the cache unit 750. The exchange is accomplished by controlling the logical states of the various signal interface lines. The cache/CPU interface includes a plurality of data to processor lines (ZDI 0-35, P0-P3), a plurality of ZAC and write data lines (ZADO 0-23, RADO 24-35, P0-P3), a processor request signal line (DREQ-CAC), a plurality of cache command lines (DMEM 0-3), a hold cache line (HOLD-C-CU), a cancel line (CANCEL-C), a flush line (CAC-FLUSH), a read word line (RD-EVEN), a read instruction buffer line (RD-IBUF), a read double (FRD-DBLE), an odd line (FODD), a plurality of instruction lines (ZIB0-35, P0-P3), a control line (DSZ), a read I-buffer data line (RD-IBUF/ZDI), a plurality of zone bit lines (DZD 0-3), a bypass cache line (BYP-CAC), a write signal line (WRT-SGN), an instruction buffer empty line (IBUF-EMPTY), an instruction buffer ready line (IBUF-RDY), an instruction buffer full line (IBUF-FULL), a CP stop line (CP-STOP), a CP control line (DATA-RECOV), a descriptor control line (FPIM-EIS), a transfer no-go line (NO-GO) and a plurality of word address lines (ZPTROUT0-1).

Instructions, cache commands and data are forwarded to the cache unit 750 via different ones of these lines. Additionally, the operation of the processor 700 is enabled or disabled by certain ones of these lines as explained herein. The description of the CPU/cache interface lines are given in greater detail herein.

    ______________________________________                                         CPU/CACHE INTERFACE LINES                                                      Designation  Description                                                       ______________________________________                                         DREQ-CAC         This line extends from processor 700                                           to cache unit 750. When the DREQ-CAC                                           line is set to a binary ONE, a ZAC                                             command is transferred to cache 750.                                           In the case of a write ZAC command,                                            write data words are transferred in                                            the one or two cycles following the                                            ZAC command and data words are sent                                            from the processor 700 through the                                             cache 750 without modification, to the                                         SIU 100.                                                      DMEM 0,1,2,3     These lines extend from the processor                                          700 to cache 750. These lines are                                              coded to designate the command that                                            the cache 750 is to execute. The                                               coding is as follows:                                                          DMEM=0000 no op No action is taken                                             and no cache request is generated.                                             DMEM=0001 Direct The direct command                                            enables the processor 700 to per-                                              form a direct transfer of an operand                                           value without action on the part of                                            the cache 750. Hence, no cache                                                 request is generated by this type of                                           command.                                                                       DMEM=0010 - Address Wraparound                                                 Command (ADD-WRAP) The address                                                 wraparound command is executed to re-                                          turn the command given to cache 750 by                                         processor 700. On the same cycle,                                              the command is given to processor 700                                          via the ZDI lines 0-35.                                                        DMEM=0100 - Load Instruction Buffer                                            Instruction Fetch 1 (LD-IBUF-IF1) The                                          load instruction buffer command is                                             used to load the address of the                                                next block of instructions into the                                            alternate instruction register RICA/                                           RICB.                                                                          There are three possible sequences of                                          operation for this command.                                              1.    In the case of a cache hit when                                                the cache 750 is not being by-                                                 passed, the block address and                                                  level stored in the cache 750 are                                              loaded into the alternate instruc-                                             tion register. A cache access is                                               made to fetch the desired instruc-                                             tion which is transferred to pro-                                              cessor 700 via the ZDI lines 0-35                                              on the subsequent T clock pulse.                                               The alternate instruction register                                             now becomes the current instruc-                                               tion register.                                                           2.    In the case of a cache miss when                                               the cache 750 is not being bypassed,                                           the block address and the level                                                designated by the round robin cir-                                             cuits are loaded into the alternate                                            instruction register. The processor                                            is turned off or held on the subse-                                            quent T clock pulse to determine                                               whether the generation of the IF1                                              command is in response to a trans-                                             fer instruction. If it is and the                                              transfer is a NO-GO, the current                                               instruction register is used to                                                access the next instruction and                                                the processor 700 is turned on. If                                             the IF1 command is caused by a                                                 transfer instruction which is a                                                GO, then cache 750 sends a memory                                              request to SIU 100 for the desired                                             block of instructions and a                                                    directory assignment is made for the                                           missing block. The instructions re-                                            ceived from memory are first written                                           into the instruction buffer and then                                           into cache. The requested instruc-                                             tion is transferred to processor                                               700 via the ZDI lines and the pro-                                             cessor 700 is turned on or released                                            on the subsequent T clock pulse.                                               The remaining instructions of the                                              block are transferred to processor                                             700 from the instruction buffer                                                via the ZIB lines.                                                       3.    When the cache is to be bypassed                                               and there is a hit, the full-empty                                             bit for that block is reset. All                                               other operations are the same as                                               in the cache miss case, except                                                 that no directory assignment is                                                made and the block is not written                                              into cache.                                                                    DMEM=0101 - Load Instruction Buffer                                            Instruction Fetch 2 (LD-IBUF-IF2) The                                          load instruction buffer command is used                                        to load the level of the second block                                          of instructions into the current in-                                           struction register. The processor 700                                          is not turned off in the case of a miss                                        condition. There are also three                                                possible sequences of operation for                                            this command.                                                            1.    In the case of a cache hit condi-                                              tion and no bypass, the level of                                               the second block of instructions                                               is loaded into the current instruc-                                            tion register.                                                           2.    In the case of a cache miss                                                    condition and no bypass, when                                                  the IF1 command was found to be                                                the result of a transfer in-                                                   struction NO-GO condition,                                                     the IF1 operation is cancelled.                                                In the case of other than a                                                    NO-GO condition, a directory                                                   assignment is made for                                                         the second block of instructions                                               and the level obtained from the                                                round robin circuits are written                                               into the current instruction regis-                                            ter. Cache 750 sends a memory                                                  request to memory for the block                                                and when the instructions are re-                                              ceived they are first written                                                  into the instruction buffer and                                                later into cache 750. When the                                                 instructions are needed, they are                                              read out from the instruction                                                  buffer and transferred to pro-                                                 cessor 700 via the ZIB lines 0-35.                                       3.    In the case of a bypass, when                                                  there is a hit condition, the full-                                            empty bit for that block is reset.                                             All other operations are the same                                              as in the case of a cache miss ex-                                             cept that there is no directory                                                assignment and the block is not                                                written into cache 750.                                                        DMEM=0110 - Load Quad The load                                                 quad command is used to load the block                                         address for data (not instructions)                                            into the alternate instruction regis-                                          ter. It is similar to the IF2 except                                           that the address and level (round                                              robin circuits provide level when a                                            cache miss condition) are written                                              into the alternate instruction regis-                                          ter. When the data is not in cache                                             750 and processor 700 requests it                                              before it is received from memory,                                             the processor 700 is held or stopped                                           until the data is received.                                                    DMEM=0111 - Pre-read (PR-RD) The                                               pre-read command is used to load cache                                         750 with data which the processor 700                                          expects to use in the near future.                                             The three possible sequences of opera-                                         tion are as follows:                                                     1.    For a cache hit, and no bypass,                                                the pre-read command is executed                                               as a no-op.                                                              2.    For a cache miss and no bypass,                                                the cache 750 generates a memory                                               request for the block and a                                                    directory assignment is made for                                               the missing block. When the data                                               is received from memory, it is                                                 written into cache. The processor                                              700 is not held for this condition.                                      3.    For a cache bypass, the pre-read                                               command is treated as a no-op.                                                 DMEM=1000 - Read Single (RD-SNG)                                               The read single command is used to                                             transfer a single data word to pro-                                            cessor 700. There are four possible                                            sequences of operation for this                                                command.                                                                 1.    In the case of a cache hit and no                                              bypass, the addressed word is                                                  read from cache 750 and trans-                                                 ferred to processor 700 on the                                                 next T clock pulse via the ZDI                                                 lines 0-35.                                                              2.    In the case of a cache miss and                                                no bypass, the processor 700 is                                                stopped and missing block is                                                   assigned in the directory. Cache                                               750 transfers the memory request                                               to main memory. The data words                                                 are written into cache as they are                                             received. When the requested                                                   data word is received, processor                                               700 is turned on upon the                                                      occurrence of the subsequent T                                                 clock pulse.                                                             3.    In the case of a cache hit and                                                 bypass, the full-empty bit of the                                              addressed block is reset and the                                               processor 700 is turned off or held.                                           The cache 750 transfers the re-                                                quest for one word to memory and                                               the processor 700 is turned on upon                                            the subsequent T clock pulse                                                   following receipt of the requested                                             data word. The data word is not                                                written into cache 750.                                                  4.    For a cache miss and bypass, the                                               same operations take place as in                                               the cache hit and bypass case                                                  with the exception that the full-                                              empty bit of the addressed block                                               is not changed.                                                                DMEM=1001 - Read Clear (RD-CLR)                                                The read clear command is used to                                              transfer a data word from memory into                                          processor 700 and also clear it out.                                           There are two possible sequences of                                            operation for this command.                                              1.    For a cache hit, the full-empty                                                bit for that block is reset and                                                processor 700 is turned off. The                                               cache 750 makes a memory request                                               for one data word. The memory                                                  clears the location. When the                                                  word is received, the cache 750                                                transfers the word to processor                                                700 and turns on the processor 700                                             on the next T clock pulse. The word                                            is not written into cache 750.                                           2.    For a cache miss, the same opera-                                              tions take place as in the cache                                               hit with the exception of no                                                   change in full-empty bits of the                                               addressed block.                                                               DMEM=1010 - Read Double (RD-DBL)                                               The read double command is used to                                             transfer two data words to processor                                           700. There are two types of read                                               double commands which differ in the                                            order in which the data words are                                              given to processor 700. When line                                              DSZ1 is a binary ZERO, the order is                                            odd word and even word. When line                                              DSZ1 is a binary ONE, the order is                                             even word and then odd word. There                                             are four possible sequences of opera-                                          tion for this command.                                                   1.    For a cache hit and no bypass, the                                             first word is transferred to pro-                                              cessor 700 on the subsequent T                                                 clock pulse via the ZDI lines 0-35.                                            On the next T clock pulse, the                                                 second data word is transferred to                                             processor 700 via the ZDI lines                                                0-35.                                                                    2.    For a cache miss and no bypass, the                                            processor 700 is turned off and a                                              directory assignment is made for                                               the block containing the addressed                                             word pair. The cache 750 transfers                                             the memory request to SIU 100 for                                              the block. As the data words                                                   are received they are written                                                  into cache. When the requested                                                 word pair is available, the first                                              word is transferred to processor                                               700 and it is turned on or re-                                                 leased on the subsequent T clock                                               pulse. The cache 750 transfers                                                 the second word to processor 700                                               on the next T clock pulse.                                               3.    For a cache hit and bypass, the                                                full-empty bit of the addressed                                                block is reset and processor 700                                               is turned off. The cache 750                                                   transfers the request to memory                                                for the two data words. As                                                     soon as the two words are available,                                           the processor 700 is turned on                                                 and the first data word is trans-                                              ferred to it on the subsequent                                                 T clock pulse. The processor 700                                               receives the second data word on                                               the next T clock pulse. The data                                               words are not written into cache.                                        4.    For a cache miss and bypass, the                                               same operations take place as in                                               the case of the cache hit and by-                                              pass, except that there is no                                                  change in full-empty bits.                                                     DMEM=1011 - Read Remote (RD-                                                   RMT) The read remote command is used                                           to circumvent normal cache read actions.                                       When the command is received, pro-                                             cessor 700 is turned off and the re-                                           quest is transferred to the main                                               memory. When the requested word                                                pair has been fetched from memory, the                                         first word is given to processor                                               700 and it is turned on the subse-                                             quent T clock pulse. The second data                                           word is transferred to processor 700                                           on the next T clock pulse. The order                                           in which the data words are trans-                                             ferred is even word and then odd word.                                         No changes are made within cache                                               750.                                                                           DMEM=1100 - Write Single (WRT-                                                 SNG) The write single command is used                                          to write data into memory. There are two                                       possible sequences of operation for                                            this command.                                                            1.    For a cache hit, the cache 750                                                 transfers the request to memory.                                               When it is accepted the data word                                              is transferred to memory. The                                                  data word is also written into                                                 cache 750.                                                               2.    For a cache miss, the same opera-                                              tions take place as the cache hit                                              except that no change is made to                                               the cache 750.                                                                 DMEM=1110 - Write Double (WRT-                                                 DBL) The write double command is used                                          to write two data words into memory.                                           This command is carried out in a                                               manner similar to the write single                                             command except that two words are                                              transferred/written rather than one                                            word.                                                                          DMEM=1111 - Write Remote (WRT-                                                 RMT) The write remote command is used                                          to circumvent normal cache write actions                                       in that when the addressed words are                                           in cache 750, they are not updated.                                            The cache 750 transfers the request                                            to memory and when accepted, the                                               two data words are transferred to                                              memory.                                                       HOLD-C-CU        This line extends from processor 700                                           to cache 750. When set to a binary                                             ONE, this control signal specifies                                             that the cache 750 is to assume a                                              HOLD state for requests or data                                                transfers.                                                    CANCEL-C         This line extends from processor 700                                           to cache 750. When set to a binary                                             ONE, this control signal indicates                                             that the cache 750 should abort any                                            processor command which is currently                                           being executed.                                               CAC-FLUSH        This line extends from processor 700                                           to cache 750. When set to a binary                                             ONE, it starts a flush of the cache                                            750 (i.e., the cache 750 is forced                                             to look empty by resetting all of the                                          full-empty bits).                                             RD-EVEN          This line extends from processor 700                                           to cache 750. When the cache makes                                             a double word request to the SIU,                                              the even word is saved in a special                                            register (REVN). When RD-EVEN line                                             is set to a binary ONE, the contents                                           of the REVN register are gated onto the                                        ZDI lines via the ZDIN switch.                                ZAD0 0-23,       These 40 unidirectional lines extend                          RAD0 24-35,      from processor 700 to cache 750. The                          P0-P3            lines are use to transfer ZAC commands                                         and write data words to cache 750.                                             When the DREQ CAC line is forced to                                            a binary ONE, ZAC command and in the                                           case of a write type of command, the                                           write data words are transferred dur-                                          ing the one or two cycles following                                            the ZAC command. The commands en-                                              coded onto the DMEM lines may or may                                           not be the same as the ZAC command.                           RD-IBUF          this line extends from the processor 700                                       to cache 750. When set to a binary ONE,                                        the line indicates that processor 700                                          is taking the instruction from the                                             instruction register RIRA. In most                                             cases, it is used to start the fetching                                        of the next instruction to be loaded                                           into RIRA.                                                    DZD 0-3          These four lines extend from processor                                         700 to cache 750. These lines trans-                                           fer odd word zone bit signals for                                              write double commands.                                        BYP-CAC          This line extends from processor 700                                           to cache 750. When set to a binary                                             ONE, this line causes the cache 750 to                                         request data words from main memory for                                        read type instructions. When a cache                                           hit occurs, the block containing the                                           requested data is removed from cache 750                                       by resetting the full-empty bit associ-                                        ated therewith. For write single or                                            double commands, the data is written in-                                       to cache 750 when a cache hit occurs.                         WRT-SGN          This line extends from the cache 750                                           to processor 700. It is used to sig-                                           nal the processor 700 during write                                             commands that the cache 750 has com-                                           pleted the transfer of ZAC commands and                                        data words to the SIU 100.                                    FPIM-EIS         This line extends from processor 700                                           to cache 750. When forced to a bin-                                            ary ONE, it signals cache 750 that                                             processor 700 is issuing an IF1 command                                        for additional EIS descriptors.                               DSZ1             This line extends from the processor                                           750 to cache 750. The state of this                                            line specifies to cache 750 the order                                          in which words are to be sent to the                                           processor 700 when a read double                                               command is performed.                                         NO-GO            This line extends from processor 700 to                                        cache 750. When forced to a binary                                             ONE, it indicates that processor                                               700 executed a transfer                                                        instruction which is a NO-GO.                                                  This signals cache 750 that it                                                 should cancel the IF1 command it                                               received when it was a miss and                                                ignore the IF2 command which is                                                currently applied to the DMEM lines.                          RD-IBUF/ZDI      This line extends from processor                                               700 to cache 750. It causes the                                                cache 750 to access the data word                                              at the address contained in the                                                alternate instruction register and                                             put this data on the ZDI lines. For                                            an outstanding LDQUAD command, the                                             cache 750 holds processor 700 when                                             line RD-IBUF/ZDI is forced to a                                                binary one.                                                   FRD-DBL          This line extends from the processor 700                                       to cache 750. This signals cache                                               750 in advance that the processor                                              700 is requesting that a read double                                           operation be performed.                                       FODD             This line extends from processor 700                                           to cache 750. This line is used in                                             conjunction with the FRD-DBL line                                              to signal the order of the words                                               being requested. When this line is a                                           binary ONE, this indicates that the                                            order is odd followed by even.                                ZDI 0-35         These 40 unidirectional lines                                 P.sub.0, P.sub.1, P.sub.2, P.sub.3                                                              extend from cache 750 to processor 700.                                        They apply data from the cache 750                                             to the processor 700.                                         ZIB 0-35         These 40 unidirectional lines extend                          P.sub.0, P.sub.1, P.sub.2, P.sub.3                                                              from cache 750 to processor 700.                                               They apply instructions to the pro-                                            cessor 700.                                                   I BUF-EMPTY      This line extends from cache 750 to                                            processor 700. When set to a binary                                            ONE, this line indicates that cache                                            750 has transferred the last instruc-                                          tion from the current instruction                                              block.                                                        I BUF-RDY        This line extends from cache 750 to                                            processor 700. When set to a binary                                            ONE, the line indicates that there                                             is at least one instruction in the                                             current instruction block in cache                                             750. The line is set to a binary                                               ZERO to indicate a non-ready condition                                         as follows:                                                              1.    Whenever the instruction address                                               switches from the last instruction                                             of an IF1 block in cache to the                                                first instruction of an IF2 block                                              not in cache and not in the                                                    IBUF2 buffer.                                                            2.    Whenever instructions are being                                                fetched from the IBUF1 or IBUF2                                                buffer and the next instruction                                                to be fetched is in a two word                                                 pair which has not been received                                               from memory.                                                  I BUF-FULL       This line extends from cache 750 to                                            processor 700. This line indicates                                             that there are at least four instruc-                                          tions in the current instruction                                               block or it has at least one instruc-                                          tion and an outstanding IF2 request.                          CP STOP          This line extends from cache 750                                               to processor 700. When forced to a                                             binary ONE state, the line signals                                             that the processor 700 is held or                                              required to wait or halt its operation.                                        In the case of a read miss condition                                           due to a processor command, processor                                          700 is held on the subsequent T clock                                          cycle pulse. When released, the                                                DATA RECOV line is forced to a binary                                          ONE to restrobe the affected processor                                         register(s). When the RDIBUF/ZDI                                               line is forced to a binary ONE before                                          the data is received from memory,                                              processor 700 is held prior to the                                             subsequent T clock pulse. When re-                                             leased, the requested data is made                                             available to processor 700 on the                                              ZDI lines and is used on the subse-                                            quent T clock pulse.                                          DATA-RECOV       This line extends from the cache 750                                           to processor 750. It is used to re-                                            strobe processor registers following                                           the stopping of the processor 700 in                                           response to the detection of a cache                                           miss condition or read bypass condi-                                           tion. At the end of the cycle in                                               which the DREQ CAC line is forced to                                           a binary ONE, the miss condition is                                            detected but processor 700 cannot be                                           stopped until after the subsequent                                             T clock pulse. Therefore, bad data/                                            instructions are strobed into the pro-                                         cessor registers from the ZDI/ZIB                                              lines. When the requested data/                                                instructions become avaiable, the                                              DATA RECOV line is forced to a                                                 binary ONE to restrobe the registers                                           which were strobed during the last                                             cache request.                                                ZPTR-OUT 0-1     These two lines extend from cache                                              750 to processor 700. These lines                                              are coded to specify the two least                                             significant bits of the address of                                             the instruction contained in the                                               RIRA instruction register or the I                                             buffer.                                                       ______________________________________                                    

General Description of Processor 700-FIG. 2

Referring to FIG. 2, it is seen that the host processor 700 includes an execution control unit 701, a control unit 704, an execution unit 714, a character unit 720, an auxiliary arithmetic and control unit (AACU) 722, and a multiply-divide unit 728, which are interconnected as shown. Additionally, the control unit 704 has a number of interconnections to the cache unit 750 as shown.

The execution control unit 701 includes an execution control store address preparation and branch unit 701-1, and an execution control store 701-2. The store 701-2 and unit 701-1 are interconnected via buses 701-3 and 701-6 as shown.

The control unit 704 includes a control logic unit 704-1, a control store 704-2, an address preparation unit 704-3, data and address output circuits 704-4, and an XAQ register section 704-5 which interconnect as shown.

As seen from FIG. 2, the SIU interface 600 provides a number of input lines to the cache unit 750. The lines of this interface have been described in detail previously. However, in connection with the operation of cache unit 750, certain ones of these lines are specially coded as follows.

1. MITS 0-3 for Reads are coded as follows:

bits 0-1=00;

bits 2-3=Transit block buffer address containing the ZAC command for current read operation.

For Write Operation bit 0-3=Odd word zone

2. MIFS lines are coded as follows:

bit 0=0;

bit 1=0 even word pairs (words 0,1);

bit 1=1 odd word pairs (words 2,3);

bits 2-3=Transit block buffer address containing the ZAC command for the data being received.

As concerns the interface lines DFS 00-35, P0-P3, these lines convey read data to cache unit 750. The lines DTS 00-35, P0-P3 are used to transfer data and commands from cache 750 to the SIU 100.

The control unit 704 provides the necessary control for performing address preparation operations, instruction fetching/execution operations and the sequential control for various cycles of operation and/or machine states. The control is generated by logic circuits of block 704-1 and by the execution control unit 701 for the various portions of the control unit 704.

The XAQ register section 704-5 includes a number of program visible registers such as index registers, an accumulator register, and quotient register. Other program visible registers, such as the instruction counter and address registers, are included within the address preparation unit 704-3.

As seen from FIG. 2, the section 704-5 receives signals from unit 704-3 representative of the contents of the instruction counter via lines RIC 00-17. Also, lines ZRESA 00-35 apply output signals from the execution unit 714 corresponding to the results of operations performed upon various operands. The section 704-5 also receives an output signal from the auxiliary arithmetic and control unit via lines RAAU0-8.

The section 704-5 provides signals representative of the contents of one of the registers included within the section as an input to the address preparation unit 704-3. The address preparation unit 704-3 forwards the information through a switch to the execution unit 714 via the lines ZDO 0-35. Similarly, the contents of certain ones of the registers contained within section 704-5 can be transferred to the execution unit 714 via the lines ZEB 00-35. Lastly, the contents of selected ones of these registers can be transferred from section 704-5 to the multiply/divide unit 728 via the lines ZAQ 00-35.

The address preparation unit 704-3 generates addresses from the contents of various registers contained therein and applies the resultant logical, effective and/or absolute addresses for distribution to other units along the lines ASFA 00-35. The address preparation unit 704-3 receives the results of operations performed on a pair of operands by the execution unit 714 via the lines ZRESB 00-35. The unit 704-3 receives signals representative of the contents of a pair of base pointer registers from the control logic unit 701 via the lines RBASA and RBASB0-1. Outputs from the multiply/divide unit 728 are applied to the address preparation unit 704-3. Lastly, the contents of a secondary instruction register (RSIR) are applied as input to the unit 704-13 via the lines RSIR 00-35.

The data and address output circuits 704-4 generate the cache memory address signals which it applies to the cache unit 750 via the lines RAD0/ZAD0 00-35. These address signals correspond to the signals applied to one of the sets of input lines ZDI 00-35, ASFA 00-35 and ZRESB 00-35 selected by switches included within the circuits of block 704-4. These circuits will be further discussed herein in greater detail.

The control logic unit 704-1 provides data paths which have an interface with various units included within the cache unit 750. As described in greater detail herein, the lines ZIB 00-35 provide an interface with an instruction buffer included within the cache 750. The lines ZDI 00-35 are used to transfer data signals from the cache 750 to the control logic unit 704-1. The ZPTROUT lines are used to transfer address information from cache 750 to unit 704-1. Other signals are applied via the other data and control lines of the cache-CPU interface 604. These lines include the CP-STOP line shown separately in FIG. 2.

As seen from FIG. 2, the control logic unit 704-1 provides a number of groups of output signals. These output signals include the contents of certain registers, as for example, a basic instruction register (RBIR) whose contents are applied as an input to control store 704-2 via the lines RBIR 18-27. The control logic unit 704-1 receives certain control signals read out from control store 704-2 via the lines CCSD0 13-31.

The control logic unit 704-1 also includes a secondary instruction register (RSIR) which is loaded in parallel with the basic instruction register at the start of processing an instruction. The contents of the secondary instruction register RSIR 00-35, as previously mentioned, are applied as inputs to the address preparation unit 704-3. Additionally, a portion of the contents of the secondary instruction register is applied as an input to the auxiliary arithmetic control unit 722 via the lines RSIR 1-9 and 24-35.

The control store 704-2 as explained herein provides for an initial decoding of program instruction op-codes and therefore is arranged to include a number of storage locations (1024), one for each possible instruction op-code.

As mentioned, signals applied to lines RBIR 18-27 are applied as inputs to control store 704-2. These signals select one of the possible 1024 storage locations. The contents of the selected storage location are applied to the lines CCSD0 13-31 and to CCSD0 00-12 as shown in FIG. 2. The signals supplied to lines CCSD0 00-12 correspond to address signals which are used to address the execution control unit 701 as explained herein.

The remaining sections of processor 700 will now be briefly described. The execution unit 714 provides for instruction execution wherein unit 714 performs arithmetic and/or shift operations upon operands selected from the various inputs. The results of such operations are applied to selected outputs. The execution unit 714 receives data from a data input bus which corresponds to lines RDI 00-35 which have as their source the control logic unit 704-1. The contents of the accumulator and quotient registers included within section 704-5 are applied to the execution unit 714 via the lines ZEB 00-35 as mentioned previously. The signals applied to the input bus lines ZDO 00-35 from the address preparation unit 704-3 are applied via switches included within the execution unit 714 as output signals to the lines ZRESA 00-35 and ZRESB 00-35, as shown in FIG. 2. Additionally, execution unit 714 receives a set of scratch pad address signals from the auxiliary arithmetic and control unit 722 applied via the lines ZRSPA 00-06. Additionally, the unit 722 also provides shift information to the unit 714 via the lines ZRSC 00-35.

The character unit 720 is used to execute character type instructions which require such operations as translation and editing of data fields. As explained herein, these types of instructions are referred to as extended instruction set (EIS) instructions. Such instructions which the character unit 720 executes include the move, scan, and compare type instructions. Signals representative of operands are applied via lines ZRESA 00-35. Information as to the type of character position within a word and the number of bits is applied to the character unit 720 via the input lines ZDB 00-07.

Information representative of the results of certain data operations is applied to the unit 722 via the lines ZOC 00-08. Such information includes exponent data and data in hexadecimal form. The character unit 720 applies output operand data and control information to the unit 722 and the unit 728 via the lines RCHU 00-35.

The auxiliary arithmetic and control unit 722 performs arithmetic operations upon control information such as exponents used in floating point operations, calculates operand lengths and pointers and generates count information. The results of these operations are applied to execution unit 714 via the lines ZRSPA 00-06 and lines ZRSC 00-06 as mentioned previously. Information signals corresponding to characters such as 9-bit characters, 6-bit characters, decimal data converted from input hexadecimal data, quotient information and sign information are applied to section 704-5 via the lines RAAU 00-08.

As seen from FIG. 2, the unit 722 receives a number of inputs. Character pointer information is applied via the lines ASFA 33-36. EIS numeric scale factor information and alphanumeric field length information are applied to the unit 722 via the lines RSIR 24-35. Other signals relating to fetching of specific instructions are applied via the lines RSIR 01-09. Exponent signals for floating point data are applied to the unit 722 via the lines ZOC 00-08 while floating point exponent data signals from unit 704-1 are applied via the lines RDI 00-08. Shift count information signals for certain instructions (e.g. binary shift instructions) are applied to the unit via the lines RDI 11-17. As concerns the input signals applied to the lines RCHU 00-35, lines 24-35 apply signals corresponding to the length of EIS instruction fields while 18-23 apply address modification signals to the unit 722.

The last unit is the multiply/divide unit 728 which provides for high-speed execution of multiply and divide instructions. This unit may be considered conventional in design and may take the form of the multiply unit described in U.S. Pat. No. 4,041,292 which is assigned to the same assignee as named herein. The unit 728 as seen from FIG. 2 receives multiplier dividend and divisor input signals via the lines RCHU 00-35. The multiplicand input signals from register section 704-5 are applied via the lines ZAQ 00-35. The results of the calculations performed by the unit 728 are applied as output signals to the lines ZMD 00-35.

As mentioned previously, the cache unit 750 transfers and receives data and control signals to and from the SIU 100 via the data interface line 600. The cache unit 750 transfers and receives data and control signals to and from the processor 700 via the lines of interface 604. Lastly, the cache unit 750 receives address and data signals from the circuits 704-4 via the lines RAD0/ZAD0 00-35.

DETAILED DESCRIPTION OF THE PROCESSOR 700

Certain ones of the sections which comprise the processor 700 illustrated in FIG. 2 will now be discussed in greater detail with respect to FIGS. 3a through 3e.

Referring to FIGS. 3a and 3b, it is seen that the processor includes two control stores: (1) the control unit control store (CCS) 704-200 which forms part of the control unit 704; and (2) the execution control store (ECS) 701-3 which is included within the execution control unit 701.

The cache oriented processor 700 of the preferred embodiment of the present invention includes a three stage pipeline. This means that the processor 700 requires at least three processor cycles to complete the processing of a given program instruction and can issue a new instruction at the beginning of each cycle. Hence, a number of program instructions may be in some stage of processing at any given instant of time.

In the preferred embodiment, the processor 700 includes the following stages: an instruction cycle (I) wherein instruction interpretation, op-code decoding and address preparation take place; a cache cycle (C) wherein access to the cache unit 750 is made ensuring high performance operation; and, an execution cycle (E) wherein instruction execution takes place under microprogram control.

As concerns control, during the I cycle, the op-code of the instruction applied via lines RBIR 18-27 is used to access a location within control store 704-2. During a C cycle, the accessed contents from control store 704-2 are applied to lines CCS D0 00-12 and in turn used to access one of the storage locations of the execution control store 701-2. During the C cycle, the microinstructions of the microprogram used to execute the instruction are read out from the execution control store 701-2 into a 144-bit output register 701-4. The signals designated MEMD0 00-143 are distributed to the various functional units of processor 700. During an E cycle, the processor executes the operation specified by the microinstruction.

Referring specifically to FIG. 2, it is seen that the control store 704-2 includes a control unit control store (CCS) 704-200 which is addressed by the op-code signals applied to the lines RBIR 18-27. The CCS 704-200, as mentioned previously, includes 1024 storage locations, the contents of which are read out into an output register 704-202 during an I cycle of operation. FIG. 6a shows schematically the format of the words stored within the control store 704-200.

Referring to FIG. 6a, it is seen that each control unit control store word includes five fields. The first field is a 13-bit field which contains an ECS starting address location for the instruction having an op-code applied to lines RBIR 18-27. The next field is a three bit field (CCSφ) which provides for the control of certain operations. The bit interpretations of this field depend upon its destination and whether it is decoded by specific logic circuits or decoded under microprogram control. The next field is a 4-bit field which provides for certain register control operations.

The next field is a 6-bit sequence control field which is coded to specify a sequence of operations to be performed under hardwired logic circuit control as well as the type of cache operation. In the present example, this field is coded as 75₈. The last field is a 6-bit indicator field which is not pertinent to an understanding of the present invention.

As seen from FIG. 3a, signals corresponding to the CCSA field of a control unit control store word are applied via a path 704-204 as an input to the execution generation circuits 701-7. Signals corresponding to the CCSR field are applied as an input to the execution unit 714 via path 704-206. Additionally, the same signals are applied as an input to the address preparation unit 704-3 via another path 704-208.

Signals representative of the sequence control field apply as an input to the sequence control logic circuits 704-101 via path 704-210. As explained herein, these circuits decode the sequence control field and generate signals for conditioning the cache unit 750 to perform the operation designated.

As mentioned previously, the execution address generation circuit 701-1 receives an input address which corresponds to field CCSA from the control store 704-2. As seen from FIG. 3b, these circuits include an input address register 701-10 whose output is connected to one position of a four position switch 701-12 designated ZECSA. The output of the switch serves as an address source for the control store 701-2. The first position of the switch 701-12 is connected to receive an address from the MICA register 701-14. The contents of register 701-14 are updated at the end of each cycle to point to the location within the ECS control store following the location whose contents were read out during that cycle.

The second position selects the address produced from the ZCSBRA branch address selector switch 701-18. The third position selects the address of the first microinstruction in each microprogram provided by the CCS control store which is loaded into the REXA register 701-10. When the CCS output is not available at the termination of a microprogram, a predetermined address (octal address 14) is automatically selected.

The first position of branch switch 701-18 receives signals corresponding to a branch address read out from store 701-2 into register 701-4 which is in turn forwarded to a return control register 701-20. The second, third and fourth positions of switch 701-18 receive signals from RSCR register 701-20, an MIC register 701-15 and the contents of a number of vector branch registers 701-36. The MIC register 701-15 stores an address which points to the microinstruction word following the microinstruction word being executed. This address corresponds to the address from switch 701-12 incremented by one by an increment circuit 701-12.

The vector branch registers include a 4-bit vector branch register 0 (RVB0), a 2-bit vector branch register 1 (RVB1) and a 2-bit vector branch register 2 (RVB2). These registers are loaded during a cycle of operation with address values derived from signals stored in a number of different indicator flip-flops and registers applied as inputs to the number of groups of input multiplexer selector circuits 701-32 and 701-34. The outputs of the circuits 701-32 and 701-34 are applied as inputs to two position selector circuits 701-30. These circuits in turn generate the output signals ZVBR0, ZVBR1 and ZVBR2 which are stored in the register 701-36.

The switch 701-36 provides an address based upon the testing of various hardware indicator signals, state flip-flop signals selected via an INDGRP field. The branch decision is determined by masking (ANDING) the selected indicator set with the INDMSKU and INDMSKL fields of a microinstruction word. If a vector branch is selected, INDMSKU is treated as 4 ZERO bits. The "OR" of the 8 bits is compared to the state defined by the TYPG and GO microinstruction fields. The hardware signals are applied via a number of data selector circuits 701-28 only one of which is shown whose outputs are in turn applied as inputs to a further five position multiplexer selector circuit 701-26. The output of the multiplexer circuit 701-26 feeds a comparison circuit which "ands" the indicator signals with the mask signals to produce the resulting signals MSKCBR0-7.

The signals MSKCBR0-7 are applied to another comparison circuit which "ands" the signals with the condition branch test signals TYPGGO to set or reset a branch decision flip-flop 701-22 which produces a signal RBDGO whose state indicates whether branching is to take place. The output signal RBDGO is applied as a control input to the first two positions of switch 701-12. When the branch test condition is not met (i.e., signal RBDGO=O), then the incremented address from the MICA register 701-14 is selected.

In some instances, as seen herein, it is not possible to test the state of an indicator on the cycle following its formation. For this reason, history registers HR0-HR7, not shown, are provided for register storage of the Group 2 indicators. The states of such stored indicators are selected and tested in a manner similar to that of the other indicators (i.e., mask fields).

Additionally, the unit 701-1 includes a number of indicator circuits, certain ones of these are used to control the operation of certain portions of the processor 700 when the strings being processed by certain types of instructions have been exhausted. These indicator circuits are included in block 701-42 and are set and reset under the control of a field within the microinstruction word of FIG. 6a (i.e., IND6 field). The bits of this field read out from the ECS output register 701-4 are applied to an RMI register 701-38 for decoding by a decoder 701-40. Based upon the state of status indicator signals received from the various processor units (e.g. 714, 720, 722, etc.), the appropriate ones of the auxiliary flip-flops are switched to binary ONE states. The outputs of these flip-flops are applied via the different positions of a 4 position switch 701-44 to the GP3 position of switch 701-26 for testing. The same outputs are applied to a second position of a ZIR switch 701-43 for storage via the ZDO switch 704-340. The ZIR switch 701-43 also receives indicator signals from an indicator register (IR) 701-41. This register is loaded via the RDI lines 18-30 and 32 in response to certain instructions.

The indicator status signals for example include the outputs of different adder circuits (AL, AXP) of the unit 720. These signals will set different ones of a number of exhaust flag flip-flops designated FE11, FE12, FE13, FE1E, FE2E, FE2 and FE3. The FE1E and FE2E flip-flops are set during any FPOA cycle of any instruction. These flip-flops in turn cause the FE11, FE12 and FE13 flip-flops to be set when the outputs from the AL or AXP adder circuits of unit 720 are generated. The setting and resetting of these indicators will be described herein in further detail in connection with the description of operation. However, the exhaust flag flip-flops pertinent to the example given herein are set and reset in accordance with the following Boolean expressions.

SET: FE1E=FPOA+IND6FLD field.

RESET: FE1E=IND6FLD field.

SET: FE2E=FPOA+IND6FLD field.

RESET: FE2E=IND6FLD field.

SET: FE11=IND6FLD field.FE1E (ALES+AXPES+DESCl.AP0-4=0)+IND6FLD field.FE1E. DESCl.(AP0-5=0+APZN+ALZN)+IND6FLD field.

RESET: FE11=FPOA+IND6FLD field.

SET: FE12=IND6FLD field.FE1E.(ALES+AXPES+FE13).

RESET: FE12=FPOA+IND6FLD field.

SET: FE13=IND6FLD field.FE1E.ALES+IND6FLD field.

RESET: FE13=FPOA+IND6FLD field.

SET: FE2=IND6FLD field.FE2E.ALES+IND6FLD field. FE2E.DESC2.(AP0-4=0+AP0-5=0+APZN+ALZN)+(IND6FLD field) FE2E.DESC2+IND6FLD.

RESET: FE2=FPOA+IND6FLD field.

SET: FE3=IND6FLD field.DESC3.(AP0-4=0+AP0-5+APZN+ALZN)+IND6FLD field.DESC3+IND6FLD.

RESET: FE3=FPOA+IND6FLD field.

Wherein IND6FLD indicates a particular code;

ALES=AL=0 or AL-C;

AXPES=AXP=0 or AXP-C;

APZN=AP0-7≦0; and,

ALZN=AL0-11≦0.

The ZCSBRA switch 701-18 is normally enabled when the branch decision flip-flop RBD was set to a binary ONE in the previous cycle. The first position selects a 13-bit branch address from the current microinstruction applied via the RCSR register 701-20. The branch address enables any one of the locations of the ECS control store to be addressed directly. The second position selects the concatenation of the 6 low order address bits from the current microinstruction applied via MIC register 701-15 and the 7 upper bits of the branch address from the current microinstruction applied via the RSCR register 701-20. This permits branches within a 64-word page defined by the contents of the MIC register 701-15 (current location+1).

The third position selects the concatenation of 4 low order bits from the RVBO vector branch register, 6 bits from the branch field of the current microinstruction stored in RCSR register and the 3 upper bits of the address stored in the MIC register. This permits 16-way branches. The fourth position selects the concatenation of the 2 low order ZEROS with 4 bits from the vector branch register RVBO with the 4 most significant bits of the branch address field of the current microinstruction and the 3 upper bits of the current address stored in the MIC register. This permits 16-way branches with 3 control store locations between each adjacent pair of destination addresses.

The fifth position selects the concatenation of 2 low order ZEROS with 2 bits from vector branch register RVB1, with the 6 bits of the branch address of the current microinstruction and the upper 3 bits from the MIC register. This permits branches with 4 possible destinations with 3 control store locations between each adjacent pair of destination addresses.

The sixth position selects the concatenation of 2 low order ZEROS with 2 bits from vector branch register RVB2 with the 6 bits of the branch address of the current microinstruction and the upper 3 bits from the MIC register. This permits 4-way branches with 3 control store locations between each adjacent pair of destination addresses.

The output of switch 701-12 addresses a specific location within control store 701-2 which causes the read out of a microinstruction word having a format illustrated in FIG. 6b. Referring to that Figure, it is seen that this microinstruction word is coded to include a number of different fields which are used to control the various functional units within processor 700. Only those fields which are related to the present example will be described herein.

    ______________________________________                                         Bits 0-1   Reserved for Future Use.                                            Bit 2      EUFMT Defines which format the EU                                              is to operate with. EUFMT-0                                                    specifies a first micro-                                                       instruction format while                                                       EUFMT=1 specifies an alter-                                                    nate microinstruction format.                                       Bits 3-5   TRL TR Low Write Control.                                                      Write control of EU temporary regis-                                           ters TR0-TR3.                                                                  OXX No change                                                                  100 Write TR0                                                                  101 Write TR1                                                                  110 Write TR2                                                                  111 Write TR3                                                       Bits 6-8   TRH TR High Write Control.                                                     Write control of EU temporary regis-                                           ters TR4-TR7.                                                                  OXX No change                                                                  100 Write TR4                                                                  101 Write TR5                                                                  110 Write TR6                                                                  111 Write TR7                                                       Bits 9-12  ZOPA ZOPA Switch Control.                                                      Selects the output of ZOPA switch.                                             (0) 0000 TR0                                                                   (1) 0001 TR1                                                                   (2) 0010 TR2                                                                   (3) 0011 TR3                                                                   (4) 0100 TR4                                                                   (5) 0101 TR5                                                                   (6) 0110 TR6                                                                   (7) 0111 TR7                                                                   (8-11) 10XX RDI                                                                (12) 1100 ZEB                                                                  (13) 1101 ZEB                                                                  (14) 1110 ZEB                                                                  (15) 1111 0 (disable)                                               Bits 13-16 ZOPB ZOPB Switch Control.                                                      Selects the output of ZOPB switch.                                  Bits 17-18 ZRESA ZRESA Switch Control.                                                    Selects the output of ZRESA switch.                                            00 ALU                                                                         01 Shifter                                                                     10 Scratchpad/RDI switch                                                       11 ZDO                                                              Bits 19-20 ZRESB ZRESB Switch Control.                                                    Selects the output of ZRESB switch.                                            00 ALU                                                                         01 Shifter                                                                     10 Scratchpad/RDI switch                                                       11 ZDO                                                              Bit 21     RSPB Scratchpad Buffer Strobe                                                  Control.                                                                       Strobes RSPB with ZRESB data.                                                  0 No strobe                                                                    1 Strobe RSPB                                                       Bit 22     RSP Scratchpad Write Control.                                                  0 Read scratchpad                                                              1 Write scratchpad                                                  Bit 23     ZSPDI Scratchpad/RDI Switch Control.                                           Selects the output of the Scratchpad/                                          RDI switch.                                                                    0 Scratchpad output                                                            1 RDI                                                               Bits 24-25 ZSHFOP Shifter Operand Switch Con-                                             trol.                                                                          Selects the left operand to the                                                Shifter.                                                                       00 ZOPA output                                                                 01 EIS output                                                                  10 0                                                                           11 Select 0 or -1 depending on bit                                             0 of right operand to Shifter.                                      Bits 24-27 ALU ALU Function Control.                                                      Selects the operation applied to the                                           two inputs (A and B) to the ALU.                                    Bits 24-29 N/a                                                                 Bits 26-31 RFU Reserved for Future Use.                                        Bits 30-31 ZALU ALU Switch Control.                                                       Selects the output of ZALU switch.                                  Bits 32-33 NXTD Next Descriptor Control.                                                  Strobes RBASB and RDESC registers.                                             00 RBASB 00                                                                       RDESC 00                                                                    01 RBASB 01                                                                       RDESC 01                                                                    10 RBASB Alt                                                                      RDESC 10                                                                    11 No strobes (default)                                             Bits 32-35 CCM Control constant field                                                     referenced by the CONTF                                                        field.                                                              Bits 34-35 IBPIPE IBUF/Pipeline Control.                                                  Selects the reading of IBUF or the                                             pipeline operation.                                                            00 No operation                                                                01 Read IBUF/ZDI (Alt)                                                         10 Type 1 Restart Release or                                                   11 Type 4 Restart Wait                                              Bits 36-37 FMTD                                                                           Selects the loading of various CU                                              registers and indicates the inter-                                             pretation to be given to the MEMADR                                            field for small CU control.                                                    00 No operation                                                                01 RADO ASFA                                                                   10 RADO ZRESB                                                                  11 RADO ASFA                                                        Bits 38-40 MEMADR Cache Control.                                                          Selects cache operations. The com-                                             plete interpretation for this control                                          is a function of the FMTD control.                                             000 No operation                                                               001 Read Sgl                                                                   010 Load Quad                                                                  011 Preread                                                                    100 Write Sgl                                                                  101 Write Dbl                                                                  110 Read Sgl Trans (for FMTD = 11                                                 only)                                                                       111 Write Sgl Word (for FMTD = 11                                                 only)                                                            Bit 41     ZONE Zone Control.                                                             Indicates zone or no zone for small                                            CU control.                                                                    0 No zone                                                                      1 Zone                                                              Bits 42-44 TYPA Type A Flag.                                                              Indicates the type A overlayed fields                                          being used.                                                                    000 Type A=0 fields                                                            .                                                                              .                                                                              .                                                                              .                                                                              100 Type A=4 fields                                                 Bits 44-46 PIPE Pipeline Control                                                          Selects the type of restart to be                                              initiated.                                                                     000 No operation                                                               001 Type 1 Restart and Release                                                 010 Type 2 Restart                                                             011 Type 3 Restart                                                             100 Type 4 Restart                                                             101 Type 5 Release                                                             110 Type 6 Restart                                                  Bits 44-47 AUXREG Auxiliary Register Write                                                Control                                                                        Selects an auxiliary register or                                               combinations to be strobed with data                                           selected by the AUXIN control field.                                           (0) 0000 No strobe                                                             (1) 0001 RRDXA                                                                 (2) 0010 R29                                                                   (3) 0011 R29, RRDXA, FRL, RID                                                  (4) 0100 RRDXB                                                                 (5) 0101 RTYP                                                                  (6) 0110 RBASA                                                                 (7) 0111 RBASA, RTYP                                                           (8) 1000 RBASB                                                                 (9) 1001 RDESC                                                                 (10) RBASA, R29, RRDXA                                              Bits 45-46 TYPB Type B Flag.                                                              Indicates the Type B overlayed fields                                          being used.                                                                    00 Type B = 0 fields                                                           .                                                                              .                                                                              .                                                                              .                                                                              11 Type B = 3 fields                                                Bit 47     RSC RSC Strobe Control.                                                        Strobes the RSC register. (Shift                                               Count)                                                              Bit 47     RSPA RSPA Strobe Control.                                                      Strobes the RSPA register.                                          Bits 47-48 N/A                                                                 Bit 47     RAAU RAAU Strobe Control.                                                      Strobes RAAU register.                                              Bits 48-49 ZLX ZLX Switch Control.                                                        Selects the output of the ZLX switch.                               Bits 48-49 ZSPA ZSPA Switch Control.                                                      Selects the output of the ZSPA                                                 switch.                                                             Bits 48-50 AUXIN Auxiliary Register Input                                                 Control.                                                                       Selects data to be strobed into                                                auxiliary register(s).                                              Bit 49     ZADSP ZADSP Switch Control.                                                    Selects the output of ZADSP switch.                                 Bits 50-52 ZSC ZSC Switch Control.                                                        Selects the output of ZSC switch.                                   Bits 50-52 ZRSPA ZRSPA Switch Control.                                                    Selects the output of ZRSPA switch.                                 Bits 50-52 ZAAU ZAAU Switch Control.                                           Bit 51     RSIR RSIR Register Strobe.                                                     Strobes the RSIR register as a function                                        of the AUXIN field.                                                 Bit 53     RDW R1DW, R2DW Register Strobe.                                                Strobes the R1DW or R2DW register as a                                         function of the RDESC register.                                     Bits 53-54 ZLNA ZLNA Switch Control.                                                      Selects output of ZLNA switch.                                      Bits 54-57 CONTF Miscellaneous Flip-Flop                                                  Control.                                                                       Selects one of four groups of control                                          flip-flops to be set or reset by the                                           control constant field (CCM). The                                              flip-flops include those of blocks                                             704-104 and 704-110.                                                Bits 55-56 ZLNB ZLNB Switch Control.                                                      Selects the output of ZLNB switch.                                  Bits 55-56 ZSPA(2) Type A=2 ZSPA Switch, RSPA                                             Register Control.                                                              Selects ZSPA switch output and strobes                                         RSPA register.                                                      Bits 57-58 ZPC ZPC Switch Control.                                                        Selects the output of ZPC switch.                                   Bits 59-62 ZXP ZXP Switch, RXP Register                                                   Bank Control.                                                                  Selects ZXP switch output and the RXP                                          register into which it will be                                                 written.                                                            Bits 59-63 ZLN(1) ZLN Switch, RLN Register                                                (Type Bank Control.                                                            A=1)                                                                           Selects ZLN switch output and the RLN                                          register into which it will be written.                             Bits 59-60 ZPA ZPA Switch Control.                                                        Selects the output of ZPA switch.                                              00 = RP0                                                                       .                                                                              .                                                                              .                                                                              .                                                                              11 = RP3                                                            Bits 61-62 ZPB ZPB Switch Control.                                                        Selects the output of ZPB switch.                                              00 = RP0                                                                       .                                                                              .                                                                              .                                                                              .                                                                              11 = RP3                                                            Bits 63-64 ZXPL ZXPL Switch Control.                                                      (Type A=0)                                                                     Selects the output of ZXPL switch.                                             00 = RXPA                                                                      .                                                                              .                                                                              .                                                                              .                                                                              11 = RXPD                                                           Bit 63     ZLN(2) ZLN Switch, RLN Register                                                (Type Bank Control.                                                            A=2)                                                                           Selects ZLN switch output and the RLN                                          register into which it will be written.                             Bits 63-66 RDIN RDI In Control.                                                           Selects the data to be strobed into                                            the RDI register and selects one of                                            the modification control fields (MF.sub.1 -                                    MF.sub.3, TAG) of an instruction word. RDI                                     strobe may also be controlled by the                                           MISCREG field.                                                      Bit 64     ZXPL(1) ZXPL Switch Control.                                                   (Type A=1)                                                                     Selects the output of ZXPL switch.                                  Bits 64-68 ZRPAC ZRPA Switch, ZRPC Switch,                                                (Type RPO-3 Register Bank Control.                                             A=2)                                                                           Selects ZRPC and ZRPA switch outputs                                           and the RPO-3 register into which the                                          ZRPA output will be written.                                        Bits 65-66 ZXPR ZXPR Switch Control.                                                      (Type A=0)                                                                     Selects the output of ZXPR switch.                                  Bits 65-66 ZXP(1) ZXP Switch, RXP Register                                                (Type Bank Control.                                                            A=1)                                                                           Selects ZXP switch output and the RXP                                          register into which it will be written.                             Bits 67-68 ZPD ZPD Switch Control.                                                        (Type A=0)                                                                     Selects the output of ZPD switch.                                   Bit 67     ZRPAC(4) ZRPA Switch, ZRPC Switch,                                             (Type RPO-3 Register Bank Control.                                             A=4)                                                                           Selects CP4 from ZRPA switch and                                               strobes the RP1 register.                                           Bit 67     TYPD Type D Flag.                                                              Type D Flag which indicates D over-                                            layed fields.                                                       Bit 68     ZRPB(4) ZRPB Switch, RP4-7 Register                                            (Type Bank Control.                                                            A=4)                                                                           Selects 0 from ZRPB switch and strobes                                         the RP4 register.                                                   Bits 68-71 MEM Cache Memory Control.                                                      Selects the cache operation in con-                                            junction with the SZ control.                                                  (0) 0000 No operation                                                          .                                                                              .                                                                              .                                                                              .                                                                              (15) 1111 Write Remote                                              Bits 68-70 IBUF IBUF Read Control.                                                        Selects the destination of IBUF data                                           when reading IBUF.                                                  Bits 69-73 AXP ZXPA Switch, ZXPB Switch,                                                  (Type AXP Adder, ZAXP Switch, RE                                               A=0) Register Control.                                                         Selects ZXPA and ZXPB switch outputs,                                          the AXP adder function applied to them,                                        and the ZAXP switch output. Also                                               strobes the RE register.                                            Bits 69-73 ZRPB ZRPB Switch, RP4-7 Register                                               (Type Bank Control.                                                            A=1)                                                                           Selects ZRPB switch output and the                                             RP4-7 register into which it will be                                           written.                                                            Bits 69-71 ZRPAC-3 ZRPA Switch, ZRPC Switch,                                              (Type RP0-3 Register Bank Control.                                             A=3)                                                                           Selects ZRPC and ZRPA switch outputs                                           and the RP-3 register into which the                                           ZRPA output will be written.                                        Bits 72-74 ZRPB(3) ZRPB Switch, RP4-7 Register                                            (Type Bank Control.                                                            A=3)                                                                           Selects ZRPB switch output and the                                             RP4-7 register into which it will be                                           written.                                                            Bits 72-73 SZ Size/Zone Cache Control.                                                    Controls cache operations in conjunction                                       with the MEM control field.                                         Bits 74-78 ZRPB(3) ZRPB Switch, RP4-7 Register                                            (Type Bank Control.                                                            A=0)                                                                           Selects ZRP switch output and the RP4-7                                        register into which it will be written                              Bits 74-78 AL ZALA Switch, ZALB Switch, AL                                                (Type Adder Control.                                                           A=1)                                                                           Selects ZALA and ZALB switch outputs                                           and the AL adder function applied to                                           them.                                                               Bit 74     TYPE Type E Flag.                                                              Type E flag which indicates the type                                           E overlayed fields.                                                 Bits 75-77 ZXP(3) ZXP Switch, RXP Register Bank                                           (Type Control.                                                                 A=3)                                                                           Selects ZXP switch output and the RXP                                          register into which it will be written.                             Bits 75-78 MISCREG Miscellaneous Register Con-                                            trol.                                                                          Selects various operations on mis-                                             cellaneous registers (e.g. RBIR, RDI,                                          RLEN, RSPP).                                                        Bits 75-78 ZDO ZDO Switch Control.                                                        Selects the output of the ZDO switch.                               Bit 78     ZIZN ZIZN Switch Control.                                                      Selects the output of ZIZN switch.                                  Bits 79-83 AP ZAPA Switch, ZAPB Switch,                                                   AP Adder Control.                                                              Selects ZAPA and ZABP switch output                                            and the AP adder function applied to                                           them.                                                               Bits 79-81 ZLN(3) ZLN Switch, RLN Register                                                (Type Bank Control.                                                            A=3)                                                                           Selects ZLN switch output and the RLN                                          register into which it will be written.                             Bits 79-83 ZLN(4) ZLN Switch, RLN Register Bank                                           (Type Control.                                                                 A=4)                                                                           Selects ZLN output and the RLN regis-                                          ter into which it will be written.                                  Bits 80-81 RAAU RAAU/RE Register Strobe.                                                  Selects the data to be strobed into                                            the RAAU and RE registers by con-                                              trolling several switches and adders                                           in the unit 722.                                                    Bits 82-83 AP(3) ZAPA Switch, ZAPB Switch,                                                (Type AP Adder Control.                                                        A=3)                                                                           Selects ZAPA and ZAPB switch outputs                                           and the AP adder function applied to                                           them.                                                               Bit 84     ZRSC ZRSC Switch Control.                                                      (Type A=0)                                                                     Selects the output of ZRSC Switch.                                  Bits 85-86 N/A                                                                 Bit 86     RLEN RLEN Strobe Control.                                                      (Type A=3)                                                                     RLEN strobes are also controlled by                                            hardware or by the MISCREG field.                                   Bit 87     FMT Format Flag.                                                               Indicates the type of format.                                       Bits 88-89 TYPF                                                                           Indicates the type of overlayed fields.                                        00 = Scratchpad Address                                                        01 = Character Unit Control                                                    01 = Multiply/Divide Control                                                   11 = N/A                                                            Bit 90     RFU Reserved for Future Use.                                        Bits 90-93 CHROP Character Unit Op code.                                                  Selects main operation to be per-                                              formed by Character Unit and the                                               interpretation to be given to the                                              CHSUBOP field.                                                                 (0) 0000 No operation                                                          (1) 0001 Load Data                                                             (2) 0010 MOP Execute                                                           (3) 0011 Compare Single                                                        (4) 0100 Compare Double                                                        (5) 0101 Load Register                                                         (6) 0110 Update CN                                                             (7) 0111 Undefined                                                             (8) 1000 Set RCH Operation A                                                   (9) 1001 Set RTF1                                                              (10) 1010 Set RTF2                                                             (11) 1011 Set RTF3 - (12) 1100 Set RCN1                                        (13) 1101 Set RCN2                                                             (14) 1110 Set Edit Flags                                                       (15) 1111 CH Unit Clear                                             Bit 90     RCH RCH Register Strobe.                                                       Strobes the OP1 RCH register.                                       Bit 90     RFU Reserved for Future Use.                                        Bits 91-97 SPA Scratchpad Address.                                                        Contains the address that may be used                                          to address the EU scratchpad.                                       Bits 91-93 N/A                                                                 Bits 94-97 CHSUBOP Character Unit Sub-Op Code.                                            Selects the detailed function of the                                           Character Unit or it may contain a con-                                        stant. The interpretation of this                                              field is a function of the CHROP                                               control as shown below.                                                        CHROP = 0000 No Operation                                                      CHSUBOP.sub.0-3                                                                XXXX No interpretation                                                         CHROP = 0001 Load Data Operation                                               CHSUBOP.sub.0-1 (Suboperation)                                                 00 OP1 Load by CN1 and TF1                                                     01 OP1 Load in Reverse by                                                      CN1 and TF1                                                                    10 OP2 Load by CN2 and TF2                                                     and Test Character                                                             11 Load Sign                                                                   CHSOBOP.sub.2-3 (Fill Control)                                                 1X Fill character loaded to                                                    ZCU                                                                            X1 Fill character loaded to                                                    ZCV                                                                            CHROP = 0010 MOP Execute Operation                                             CHSUBOP.sub.0-1 (Suboperation)                                                 00 MOP set, by CN2                                                             01 MOP Execute                                                                 10 Undefined                                                                   11 Undefined                                                                   CHUBOP.sub.2-3                                                                 XX No interpretation                                                Bit 94-97  CHSUBOP (continued)                                                            CHROP = 0101 Load Register Operation                                           CHSUBOP.sub.0-1 (Selects output of RCH)                                        CHSUBOP.sub.2-3 (Selects output of ZOC                                         switch)                                                                        CHROP = 1011 Set RTF3 Operation                                                CHSUBOP.sub.0-1 (Selects data to be                                            inspected for 00, indicat-                                                     ing a 9-bit character.)                                                        CHSUBOP.sub.2-3 (Constant Field)                                               CHROP = 1110 Set Edit Flags Operation                                          CHSUBOP.sub.0-3 (Constant selecting flags                                      to be set)                                                                     1XXX Set ES (End supression)                                                   X1XX Set SN (sign)                                                             XX1X Set Z (zero)                                                              XXX1 Set BZ (Blank When Zero).                                      Bits 94-97 RFU Reserved for Future Use.                                        Bits 97-97 N/A                                                                 Bit 98     TYPG Type G FLAG.                                                              Indicates the type of overlayed fields.                                        0 = BRADRU field                                                               1 = IND6 field                                                      Bit 99     GO State of Conditional Branch                                                 Test.                                                               Bits 99-106                                                                               BRADRU Branch Address Upper.                                        Bits 99-106                                                                               IND6FLD Indicator Control.                                                     Selects an indicator.                                               Bits 99-106                                                                               Bit 99 = 0 specifies a change indica-                                          tors instruction.                                                              Bit 99 = 1 specifies a set/reset indi-                                         cators instruction (set or reset                                               indicated by X bit 0 or 1 respectively).                                     Bits 100-104                                                                            105=1     106=1                                              ______________________________________                                                  0000                                                                           .                                                                              .                                                                              .                                                                              .                                                                              1100X    Exhaust 1 Exhaust 2                                                   1101X    Exhaust 3 N/A                                                         1110X    Exhaust 1 Exhaust 2                                                            Eff.      Eff.                                               Bits 107-112                                                                              BRADRL BRANCH ADDRESS LOWER.                                                   Contains lower portion of an ECS address                                       used for branching.                                                 Bit 113    EXIT Selection of Exit Switch con-                                             trol.                                                                          Selection of Exit indicates end of                                             microprogram.                                                       Bits 114-116                                                                              ZCSBRA ZCSBRA Switch Control.                                                  Defines the position to be selected                                            in a Control Store Branch Address                                              Switch.                                                             Bits 117-118                                                                              N/A                                                                 Bits 119-123                                                                              INDGRP Conditional Branch Indicator                                            Group Control.                                                                 The first two bits (119-120) select the                                        "group" of microprogram indicators.                                            The last three bits (121-123) select                                           the "set" of indicators within each                                            "group".                                                            Bit 124    TYPH Type H field.                                                             Indicates the type H overlayed fields.                                         0 = INDMSKU                                                                    1 = VCTR field                                                      Bits 125-128                                                                              INDMSKU Conditional Branch Indicator                                           Mask Upper.                                                                    Contains the upper 4 bits of the indi-                                         cator mask in type H = 0 field.                                     Bits 125-129                                                                              VCTR Vector Select.                                                            Selects the branching vectors to be                                            strobed into the RVB0, RVB1, and RVB2                                          registers. The most significant bit                                            (125) determines which of two groups                                           0 or 1, 2, or 3 and 4 or 5 will be                                             strobed into the RVB0, RVB1, and RVB2                                          registers respectively. The remaining                                          3 bits select the vector within each                                           group.                                                              Bits 129-132                                                                              INDMSKL Conditional Branch Indicator                                           Mask Lower.                                                                    Contains the lower 4 bits of the                                               indicator mask.                                                     Bits 133-135                                                                              N/A                                                                 Bits 136-139                                                                              CNSTU Constant Upper.                                                          Contains the upper 4 bits of the con-                                          stant field.                                                        Bits 140-143                                                                              CNSTL Constant Lower.                                                          Contains the lower 4 bits of the con-                                          stant field.                                                        ______________________________________                                    

Control Logic Unit 704-1

This unit includes the sequence decode logic circuits 704-100 as mentioned whose outputs feed a plurality of I cycle control state flip-flops of block 704-102. These flip-flops in response to signals from the circuits 704-100 as well as microinstruction signals from register 701-4 (DMEMRO38-40 which corresponds to the mem address field MEMADR of FIG. 6b) generate the various required I cycle control states required for the execution of program instructions. It is assumed that block 704-102 also includes gate circuits which generate register hold signals (HOLDE00) which are distributed throughout the processor 700.

As seen from FIG. 3c, the I cycle control state flip-flops receive control input signals via control lines including a line CPSTOP00 from cache unit 750. As explained herein, the state of the CPSTOP00 line determines whether processor operation continues in that when the line is forced to a binary ZERO, the hold or enabling signals for the I cycle control state flip-flops and other storage registers are also forced to ZEROS. The hold signals corresponding to signals [HOLDI00 and [HOLDE00 operate to hold or freeze the state of the processor 700. Since no incrementing of the control store address can take, the ECS control store reads out the same microinstruction word. The signals [HOLDI and [HOLDE are set in accordance with the following Boolean expressions: [HOLDI=CACHE HOLD+TERMB (DREQ-IF-DIR)+HOLD REL wherein the state of signal CACHE HOLD corresponds to the state of signal CPSTOP, the states of signals TERMB (DREQ-IF-DIR) are binary ONES during control state FPOA when the cache command specifies an I fetch or direct operation and the signal HOLD REL is a binary ONE until switched to a binary ZERO by the generation of a microprogram release signal; and [HOLD E=[HOLD I.

As seen from FIG. 3c, signals corresponding to the I cycle control states are applied as inputs to a plurality of control flip-flops of block 704-104, decoder circuits of block 704-106, a number of control logic circuits of block 704-108 and to a plurality of control flag indicator flip-flops of block 704-110. It is also seen that the various indicator flip-flops of block 704-110 also receive microinstruction input signals via lines MEMD054-57 from execution control unit 701-4.

As seen from FIG. 3c, signals generated by the hardware control logic circuits 704-108 fall into one of three groups as a function of the units whose operations are being controlled. That is, the groups are instruction buffer control, hardware control and hardware memory control.

In each case, each group of signals is ored together with equivalent signals generated by other sources and then decoded. The other sources correspond to fields within the two different formats of the microinstruction word of FIG. 6a which are loaded into RCSR register 704-112 from the ECS output register 701-4.

One field corresponds to bits 32-83 of one format (large CU) and another field (short CU) corresponds to bits 32-41 of another format. These fields are decoded by a decoder 704-114 into the sets of bits indicated and combined within the decoders 704-116, 704-124, 704-126 and 704-128 as shown. Further decoding is done by the circuits of blocks 704-118, 704-135 and 704-120. The results of decoding such fields are either distributed throughout processor 700 or are stored in an RMEM register 704-130, an RSZ flip-flop 704-132, an FREQDIR flip-flop 704-136 and an FREQCAC flip-flop 704-134.

Additional decoding of the large and short CU fields and signals from the I cycle state circuits of block 704-112 is done via decoders 704-106 and 704-107. The decoder 704-106 generates control signals for loading different ones of the registers and for enabling various multiplexer/selector switches within the processor 700. The decoder 704-107 operates to generate signals for setting and resetting a pair (RBASB) of base pointer B flip-flops 704-144. Other combinations of these signals are used to set and reset the descriptor number flip-flops of blocks 704-140 and 704-142.

As seen from FIG. 3c, the decoder 704-116 receives a control signal [EXH00 generated by the decoder circuits of block 704-117. These circuits receive signals from the RDESC register 704-140 and signals from the exhaust flip-flops of block 701-1. In accordance with the states of these signals, the circuits force signal [EXH000 to a binary ZERO to inhibit the generation of a cache memory command upon the occurrence of an exhaust condition. The signal [EXH000 is generated in accordance with the following Boolean expression:

[EXH000=DESCO.FE11+DESC1.FE2+DESC2.FE3.

The flip-flop FNUM is normally set in response to the CCS-OP field of the microinstruction word. When set to a binary ONE, this indicates that the descriptor being processed is a numeric type.

The different flip-flops of block 704-104 will now be discussed in greater detail. In greater detail, the flip-flop FCHAR provides certain changes in the control of address generation. When the FCHAR flip-flop is set to a binary ONE during the processing of a load type instruction specifying character modification, then the contents of the RDI register are not changed under hardware contro. This allows the RDI register to be loaded with data under microprogram control prior to starting the pipeline. Also, if the FCHAR flip-flop is set to a binary ONE during a store type instruction specifying character modification, then the execution address for this instruction is modified under hardware control to point to a unique address of the microinstruction sequence in the ECS control store that is to process this type of instruction.

The flip-flop FDT-FOUR provides additional control on the readout of the address register (ZAR₀₋₁₉) of block 704-304. Flip-flop FADR-WD provides additional control for the ZDO switch 704-340. When this flip-flop is set to a binary ONE, then the ZAR position of the ZDO switch is forced to select a word address. The flip-flop FADR-B provides additional control for the ZDO multiplexer switch. When set to a ONE, then the ZAR position of the ZDO switch is forced to select a byte address. The flip-flop FNUM is normally set in response to the CCS-OP field of the microinstruction word. When set to a binary ONE, this indicates that the descriptor being processed is a numeric type. The flip-flop FIG-LEN provides additional control over the loading of registers within the unit 722 (length registers) and over memory operations. When set to a binary ONE, the RXP and RLN registers within unit 722 are not loaded from the RSIR register 704-154 during certain processor control states FPOP.

The FINH-ADR flip-flop inhibits the operation of the address prparation unit 704-3. When set to a binary ONE, an address cycle (FPOA/FPOP) consists of adding the contents of a temporary effective address register REA-T+ZERO. The register REA-T will have been loaded with the address prior to doing a FPOA/FPOP cycle. The FABS flip-flop enables the generation of absolute addresses. When set to a binary ONE, a 24-bit absolute address is used. As concerns the flag or indicator flip-flops of block 704-110, flip-flop FID when set to a binary ONE provides an indication that indirect address modification during an instruction is required on the descriptor loaded into the RSIR register.

The FRL flip-flop when set to a binary ONE indicates that the length is specified in a register associated with the instruction loaded into various instruction registers. The three flip-flops FINDA, FINDB and FINDC provide indications used in processing memory type instructions. Flip-flop FINDA is set to a binary ONE when length is specified in a register or when flip-flop FAFI is set to a ONE. Flip-flop FINDB is set to a binary ONE when the descriptor does not include nine bit characters. The flip-flop FINDC is set to a binary ONE when the descriptor does include six bit characters.

The FAFI flip-flop is set to a binary ONE when the processor circuits detect that indicator bit 30 of IR register 701-41 was set to a binary ONE during the execution of an EIS instruction indicative of a mid instruction interrupt (required to adjust pointer and length values because of interrupt). The FTRGP, TTNGO and FTRF-TST flip-flops are set to binary ONES in conjunction with transfer type instructions. More specifically, the FTRGP flip-flop provides a microprogram indication of being set to a binary ONE when the processor circuits detect the read out of a transfer type of instruction during the execution of an execute double (XED) or repeat (RPTS) instruction. The FTNGO flip-flop provides a microprogram indication of being set to a binary ONE when the condition of transfer signalled by the execution control unit 701 was transfer NO GO (i.e., transfer did not take place). The output of this flip-flop is applied to the NO GO line of interface 604. The FTRF-TST flip-flop of this group indicates when set to a binary ONE that the previous instruction executed by processor 700 was a transfer type instruction and that the curret I cycle is to be executed conditioned upon the presence of a transfer GO (TRGO) signal from control unit 701.

Additionally, the circuits of block 704-110 include a number of flip-flops used in performing indirect addressing operations under hardwired control for other than EIS instructions. These include FIR, FIRT, FIRL and FRI flip-flops which are switched to binary ONES as functions of the different types of indirect address modifications required to be performed. For example, the FRI flip-flop signals a register then indirect address modification and is switched to a binary ONE when a register indirect (RI) indicator is a binary ONE. The FIR flip-flop is switched to a binary ONE when an indirect then register (IR) indicator is a binary ONE. This flip-flop signals the beginning of an indirect then register address modification. The FIRL flip-flop is switched to a binary ONE when an indirect then tally indirect (IT-I) indicator is a binary ONE. This flip-flop signals a last indirect operation. Another flip-flop TSX2 provides an indication used in processing transfer and set index instructions while a STR-CPR flip-flop is used during the processing of store instructions.

As seen from FIG. 3c, the outputs from the control flag flip-flops of block 704-110 are applied as inputs to the branch indicator circuits of block 701-1. Also, output signals from the control flag flip-flops are also applied as inputs to the I cycle flip-flops of block 704-102.

Register Section 704-150

As seen from FIG. 3c, the control logic unit 704-1 further includes a register section 704-150. This section contains the basic instruction register (RBIR) 704-152, the secondary instruction register (RSIR) 704-154, a base pointer A register (RBASA) 704-156 used for selecting one of the address registers RAR0 through RAR7 of block 704-304, a read index register A (RRDXA) 704-158 used for selection of index registers included within section 704-5 (not shown) and for selection of outputs from the ZDO multiplexer switch 704-340, a read index A save (RRDXAS) register 704-159, and a descriptor type register (RTYP) 704-160 indicating the type of data characters being pointed to by the descriptor value (e.g. 9-bit, 6-bit, 4-bit). The section 704-150 further includes a 1-bit instruction/EIS descriptor register designated R29 of block 704-162. The state of this bit in conjunction with the contents of the RBAS-A register 704-158 are used to select the particular address register used for address preparation. When register R29 of block 704-162 is set to a binary ZERO, this indicates that none of the address registers of block 704-304 are used during address preparation. The last registers of section 704-150 include the data in register (RDI) of block 704-164 and a read index register B (RRDXB) pointing to registers used by execution unit 714.

As seen from FIG. 3c, the RBIR register 704-152 is loaded via a two position switch 740-170 connected to receive signals from the sources indicated (i.e., a switch ZIB-B 704-172 and lines ZDI 0-35). The RSIR register 704-154 similarly receives signals from the ZDI lines and switch 704-172. The RBASA register 704-156 receives signals from the ZDI line 0-2 in addition to a further switch ZBASA of block 704-174. The RRDXA register and RTYP register receive signals from the ZDI lines as well as switches 704-176 and 704-178 as shown. Also, the RRDXA register receives signals from the RRDXAS register 704-159.

The switch 704-172 is a two position switch which receives inputs from the switches ZIB and ZRESB from the cache unit 750 and execution unit 714 respectively. The switch 704-174 is a three input switch which receives two inputs from the execution unit 714 and the output of the ZIB switch of cache unit 750.

Switch 704-176 is a four input switch which receives two of its inputs from the execution unit 714 and a single input from cache unit 750. The first position of the ZRDXA switch 704-176 selects the output of a ZRDXM switch 704-185. One position of this switch provides a tag field value from bit positions 5-8, 14-17, and 32-35 of the RBIR register 704-152 and bit positions 32-35 of the RSIR register 704-154 selected from ZIDD switch 704-180 and a two position ZMR switch 740-176.

The second position of switch 704-185 provides a constant value from the output of the ECS output register 704-1 (CCM field 32-34). The signals from the lines ZIDD 27-35 are applied as inputs to control flag flip-flops of block 704-110. The switch 704-178 receives an input from the control store 704-2, an input from cache unit 750 and an input from execution unit 714.

The data input register 704-164 receives a series of input signals from a ZIDD switch 704-180 which connects in series to a ZDIA switch 704-181 whose output provides one input of a further switch 704-182 which directly loads into the RDI register 704-164. The ZDIA switch 704-181 provides a further input to a three input switch 704-183 which receives the other inputs indicated from cache unit 750 and execution unit 714.

The ZIDD switch 704-180 receives an effective address via switch 704-186 from the address preparation unit 704-3 as well as inputs from the RBIR register 704-152, the RSIR register 704-154 and a two position ZMF switch 704-187. The positions 18 through 35 of the REA position of switch 704-180 are derived from the ZDIA switch 704-181 as shown. The ZDIA switch 704-181 receives signals from the ZDI lines 0-35, a constant value generated from the inputs to a first switch position in addition to signals from the output of the ZIDD switch 704-80 and the ZRESB switch in execution unit 714. The switch 704-182 receives the output of the ZDIA switch and signals from the ZDI lines 0-35. The RRDXB register 704-189 is loaded by a three position switch 704-188. The switch receives via a first position signals from a RREG register included in the execution unit, a constant value from control store 701-2 via a second position and signals from the ZIDD switch via a third position.

The section 704-150 further includes a two position switch 704-185 and a scratchpad pointer register 704-186 whose output is used by the AACU 722 to form addresses for access to the scratchpad memory of the EU 714. The first switch position provides a constant value and is selected under hardware control (FPOA.R29). The second switch position applies as an output the contents of the RBASA register 704-156. This position is selected under both hardware and microprogram control (i.e. FPOA.R29 or MISCREG field).

It will be appreciated that the required timing signals for operating section 704 as well as other sections of processor 700 and cache unit 750 are provided by centrally located clock circuits. For example, in the preferred embodiment of FIG. 1, the clock circuits are located within the input/output processor system. Such clock circuits can be considered as conventional in design and can comprise a crystal controlled oscillator and counter circuits. The timing or clocking signals from such clock circuits are distributed in a conventional manner to the various portions of the system of FIG. 1 for synchronized operation. From such timing signals, circuits within processor 700 derive additional clocking signals as required. This will be described in greater detail with respect to the cache unit 750 of FIG. 4.

Address Preparation Unit 704-1

The address preparation unit 704-3 includes a number of registers and adders. The registers include a number of base registers (i.e., TBASE0 through TBASEB) of block 704-300 used for storing descriptor values of an instruction, a pair of temporary effective address registers (TEA0, TEA1) and a pair of instruction counters (ICBA, ICBB) included within block 704-302 used for addressing the instruction buffer and eight address registers (RAR0 through RAR7) of 704-304 used during address preparation operations. The unit 704-3 also includes an instruction counter 704-310.

The adders include adder 704-312 used to update instruction counter 304-310 via switches 704-311 and 704-314 and a pair of adders 704-320 and 704-322. The adder 704-322 is used to generate an effective address value which is stored in a register 704-342 applied as an input of the control unit 704-1. The effective address is generated from a number of sources which include ZY switch 704-326 whose output is applied via a number of AND gates of block 704-327, selected address registers of block 704-304 or selected temporary address registers TEA0 and TEA1 of block 704-302 applied via another switch 704-328 or the index address signals ZX0-20 from unit 704-5. Additionally, adder 704-322 is used to update the contents of the instruction counter of the cache instruction buffer.

As seen from FIG. 3d, the outputs from adder 704-322 are also applied as an input to the adder 704-320. The adder 704-320 is used to combine base value stored in any one of the temporary base registers TBASE0 through TBASEB with the address signals ACSOS0-19 from adder 704-322. The resulting bits are applied as an input to a further adder network 704-320 which generates a logical address which is applied to the lines ASFA0-36 via an adder 704-321. This adder sums the operand inputs together with the carry inputs from blocks 704-300 and 704-320. The effective address is used to obtain an absolute address when the system is operated in a paged mode. Since this operation is not pertinent to the present invention, it will not be discussed further herein. For further information regarding such address development, reference may be made to U.S. Pat. No. 3,976,978.

The temporary base registers of block 704-300 are loaded via a switch 704-332. The switch receives an input from the execution unit 714 and the output from block 704-300. The execution unit 714 applies further inputs to the registers of block 704-302 via a switch 704-334 as well as to the address registers of block 704-304. An output multiplexer (ZD0) switch 704-340 enables the selection of the various registers within the address preparation unit 704-3 and unit 704-5 for transfer of their contents to the execution unit 714 via lines ZDO 0-35. Also, the ZDO switch 704-340 enables the contents of various ones of the registers and control flip-flops of unit 704-1 to be read out via a fourth position (ZDO-A). The fifth position enables the states of various indicators within the control store circuits of block 701-1 to be selected for examination.

Data/Address Output Section 704-4 FIG. 3e

The section 704-4 includes the registers and switches used for transferring commands and data to the cache 750. Such transfer operations normally require at least two cycles, one for sending an address and another for sending the data. Bits 5-8 of a command word are derived from the output of a four position switch 704-40. This switch receives a first constant value via a first position, the contents of a RZN register 704-42 via a second position, a second constant value via a third position and a third constant value via a fourth position.

Bits 1-4 of a command are applied by the circuits of block 704-1 to an OR gate circuit 704-44 together with bits 5-8. The OR gate 704-44 also receives via a ZADO switch 704-46 bits 1-8 of an RADO register 704-48. The RADO register 704-48 is an address and data out register which receives via a first position of a ZADOB switch 704-48 a logical (virtual) address from address preparation unit 704-3 via the lines ASFA0-35 and data output signals from the EU 714 via lines ZRESB0-35. The positions of the ZADOB switch 704-48 are under the control of the FMTD field for small CU format and the RADO field in the case of large CU format.

As seen from the Figure, either the ZZN1-8 bits or the ZADO bits 1-8 are applied as outputs to the RADO/ZADO lines as a function of the state of control signal [RADO-ZADO. Bits O and I are always binary ONES while bits 10-35 are furnished by the RADO register 704-46.

For additional information regarding the remaining sections of processor 700 as well as the sections of FIGS. 3a through 3e, reference may be made to the copending applications referenced in the introductory portion of this application.

CACHE UNIT 750 - FIG. 4 General Description

The cache unit 750 is divided into five primary sections: a transit buffer and command queue section 750-1, a cache section 750-3, a directory and hit control section 750-5, an instruction buffer section 750-7 and an instruction counter section 750-9.

Transit Buffer and Command Queue Section 750-1

The transit buffer and command queue section 750-1 includes as major elements a four word write command buffer 750-100 and a four word transit block buffer read command buffer 750-102 which are addressed via a pair of counter circuits 750-104 and 750-106 in addition to a command queue 750-107 with associated in and out address pointer and compare circuits of blocks 750-108 through 750-110. The write buffer 750-100 provides storage for two write single or one write double command while the transit block 750-102 provides storage for up to four read type commands. The transit block buffer 750-102 also stores information associated with such read commands used in controlling the writing of memory data words into assigned areas (i.e., levels) of cache section 750-3. The four registers allow up to four memory reads to be in progress at any given time.

Section 750-1 also includes a control section 750-112. This section includes sets of different control circuits such as the command decoder and control circuits of blocks 750-113 and 750-114, the interface control circuits of blocks 750-115 and 750-116 and hold control circuits of block 750-117.

The circuits of blocks 750-113 and 750-114 decode the signals applied to the DMEM lines representative of commands transferred by processor 700 via the RADO/ZADO lines of interface 604 and generate the control signals for making entries in the command queue 750-107, incrementing and setting values into the in pointer and out pointer circuits of blocks 750-108 and 750-109. Also, the circuits generate control signals for storing commands into either write buffer 750-100 or transit block buffer 750-102.

The interface control circuits of blocks 750-115 and 750-116 generate signals for controlling the transfer of data signals received from SIU 100 into section 750-7 and for commands including the transfer of such commands to the SIU respectively. The hold circuits of block 750-117 which receive signals from decoder circuit 750-113 generate control signals for holding the execution of commands in appropriate situations (e.g. directory section busy) and controlling the loading of data into section 750-7.

As seen from FIG. 2, the transfer of write command control words proceed from buffer 750-100 via the third position of four position (ZDTS) switch 750-118, a data register 750-119 and the first position of two position switch 750-120. The write data words are transferred from buffer 750-100 to SIU 100 via a write data register 750-121 and the second position of switch 750-120. The RWRT position of switch 750-120 is selected for one (write single command) or two (write double command) clock intervals following receipt of a signal from SIU 100 via the ARA line made in response to a signal placed on line AOPR by cache 750 for transfer of the write command. Read commands are transferred from the read command portion of transit block buffer 750-102 to SIU 100 via the fourth position (ZTBC) of the ZDTS switch 750-118, register 750-119 and the first position of switch 750-120.

The multiport identifier lines MITS receive zone bit signals via a RMITS register 750-124 and a two position switch 750-125 for the second data word in the case of a write double command. As seen from the Figure, this switch receives signals from command queue 750-107 and processor 700. That is, when cache 750 issues a read command, transit block number signals from queue 750-107 are loaded into bit positions 2 and 3 of RMITS register 750-124.

The transit block number signals are returned by SIU 100 on the MIFS lines with the read data word. These signals are loaded into an RMIFS register 750-127 via a multiposition switch 750-126. Thereafter, the contents of bit positions 2 and 3 are applied via the first position of a two position switch 750-128 to a pair of address input terminals of transit block buffer 750-102. A second RMIFSB register 705-129 primarily provides temporary storage of the transit block number signals for multiword transfers (i.e., quad read commands).

The output signals from switch 750-128 are also applied to the control input terminals of a four position ZTBA switch 750-130 for selecting the appropriate address signals to be applied to cache section 750-3 for storage of the data words. The address contents of the transit block buffer 750-102 are also applied to one set of input terminals of a predetermined one of a group of compare circuits 750-132 through 750-135 for comparison with the address portion of a next command applied to a second set of input terminals of the comparator circuits via the RADO/ZADO lines. The result of the comparisons generated by a NAND gate 750-136 is applied to the hold control circuits of block 750-117.

As seen from FIG. 4, the zone bit signals of the ZAC command applied to the ZADOB lines 5-8, in the case of a write single command, or for the even word of a write double command, are loaded into a RZONE register 750-140 when the write command is loaded into write command data buffer 750-100. The output of RZONE register 750-140 is applied to the first position of a two position ZONE switch 750-144. The zone bit signals applied to the lines DZD0-3 by processor 700 for the odd word of a write double command, are loaded into a RDZD register 750-142. The output of RDZD register 750-142 is applied to the second position of ZONE switch 750-144. The output signals ZONE0-3 are applied to the circuits of section 750-9 for controlling the writing of processor data into cache 750-300 as explained herein.

Cache Section 750-3

The section 750-3 includes a cache store 750-300 having 8192 (8K) 36-bit word locations organized into 128 sets of eight, eight word blocks. The unit 750-300 is constructed from bipolar random access memory chips, conventional in design.

The cache storage unit 750-300 is addressed by a 10-bit address RADR 24-33 applied via any one of a number of 4×4 crossbar switches (e.g. 750-302a), conventional in design and the address registers associated therewith. As seen from the Figure, the crossbar switch receives address signals from several sources which include section 750-5, ZTBA switch 750-130 and section 750-7. The address signals appearing at the output of the crossbar switch are temporarily stored in the associated address register and applied to the address input terminals of cache storage unit 750-300.

During a write cycle of operation, the four sets of write control signals (WRT00100-WRT70100 through WRT03100-73100) generated by section 750-9, are applied to the cache storage unit 750-300 and are used to apply or gate clocking signals to the write strobe input terminals of the memory chips. The enables from one to four bytes of either a processor 700 data word from the ZADO/RADO lines or a memory data word from section 750-7 to be written into the addressed one of eight levels of cache storage unit 750-300. For processor data, the write signals are generated by decoding signals ZONE0-3 from switch 750-144. For memory data words, all of the zone signals are forced to binary ONES.

The appropriate level is established by the states of signals RTBLEV0100-2100 from transit block buffer 750-102 when writing memory data and by the hit level detected by directory circuits of block 750-512 when writing processor data. These signals are decoded by a decoder circuit 750-303 when enabled by a signal ENBMEMLEV100 from section 750-9.

During a read cycle of operation, the 36-bit word of each of the eight blocks (levels) is applied as an input to a 1 of 8 ZCD switch 750-306. The selection of the appropriate word is established by the states of a set of hit level signals ZCD010-210 generated by section 750-5. These signals are applied to the control input terminals of ZCD switch 750-306.

As seen from the Figure, the selected word is applied to a pair of registers 750-308 and 750-310, a 1 of 8 ZDI switch 750-312 and a 1 of 4 ZIB switch 750-314. The RIRA and RIRB registers 750-308 and 750-310 apply their contents to different positions of the ZIB and ZDI switches 750-312 and 750-314. The ZIB switch 750-314 selects instructions which are applied to the instruction bus (ZIB) of processor 700 while the ZDI switch 750-312 selects data or instructions which are applied to the data in bus (ZDI) of processor 700.

In addition to applying instruction word signals read out from cache 750-300, the ZIB switch 750-314 also applies instruction word signals received from section 750-7 to processor 700. The ZDI switch 750-312 also applies data signals received from the ZCDIN switch 750-304 and section 750-7 to processor 700. The states of the control signals [ZID010-110 and [ZDI010-210 applied to the control input terminals of switches 750-314 and 750-312 select the sources of instruction and data words to be transferred to processor 700 by such switches. The control signals are generated by the circuits of section 750-9.

In greater detail, the [ZIB010-110 signals are coded to select position #2 of switch 750-314 for a first instruction transfer in response to the detection of a directory hit for an I fetch 1 command or a directory hit for an I fetch 2 command following an I fetch 1 command to the last word in a block. The control signals are coded to select the RIRA position #1 for subsequent instruction transfers following a directory hit generated in response to an I fetch 1 or I fetch 2 command.

Where the I fetch 1 or I fetch 2 command results in a directory miss, the [ZIB010-110 signals are coded to select position #3 of ZIB switch 750-314 for transfer of instruction words received from section 750-7.

As concerns the ZDI switch 750-312, the ZCD position #1 is selected in response to the detection of directory hits and signals applied to the RDIBUF/ZDI line in response to a directory hit generated for a LDQUAD command. Memory data words are transferred to processor 700 via the ZDIN position #3 of the switch 750-312 following a directory miss. Following holding processor 700 for an instruction fetch from main memory, the signals [ZDI010-210 are coded to select the ZDIN position of switch 750-312 for transfer of the first instruction upon its receipt by section 750-7. The remaining instructions are transferred via ZIB switch 750-314.

The ZCDIN position #2 of switch 750-312 is used for diagnostic purposes to transfer signals from the ZADO-B/RADO lines. The remaining positions of ZDI switch 750-312 are used for display purposes (i.e., positions RIRB, ZRIB and RIRA). Also, position RIRB is selected to transfer data words to processor 700 in the case of LDQUAD command when there is a directory hit.

Directory and Hit Control Section 750-5

This section includes an eight level control directory 750-500 and eight level set associative address directory 750-502. The directory 750-502 contains 128 locations, each location containing a 14-bit associative address for each level. A four position ZDAD switch 750-530 provides the random access memory (RAM) addresses for addressing directories 750-500 and 750-502 in addition to cache storage unit 750-300.

During a directory search cycle of operation, switch 750-530 under the control of signals SELZDADC0100-1100 generated by circuits within a block 750-526 selects RADO position 0. This applies the 14-bit address signals of a ZAC command from lines RADO 24-33 from processor 700 to the output terminals of the ZDAD switch 750-530. These signals are applied to the address input terminals of directories 750-500 and 750-502. During the search cycle, the contents of eight block/level addresses are read out and applied as one input of each of a group of eight comparator circuits 750-536 through 750-543. Each comparator circuit compares its block/level address with bits 10-23 of the ZAC command to determine a hit or miss condition. The results generated by the circuits 750-536 through 750-543 are applied to corresponding inputs of a group of AND gates 750-545 through 750-552. Each comparator circuit is made up of four sections, the results of which are combined in one of the AND gates 750-545 through 750-552. The final result hit signals ZHT0100 through ZHT7100 are applied as inputs to hit/miss network circuits of block 750-512 as explained herein.

The ZAC address signals are also saved in an RDAD register 750-532 when no hold condition is detected (i.e., signal [HOLD-DMEM from 750-112 is a binary ZERO). During the directory assignment cycle following the search cycle which detected a miss condition, signals SELZDADC0100-100 select RDAD position 1 of ZDAD switch 750-530. Also, a RDRIN register 750-534 is loaded with the 14-bit associative address signals from the ZADO-B lines 10-23 when the directory search cycle is completed for writing into the directory 750-502.

The control directory 750-500 also includes 128 locations, each having a predetermined number of bit positions for storing control information. Such information includes the full-empty (F/E) bits for the eight levels and round robin (RR) count bits in addition to parity check bits (not shown).

The full-empty bits indicate whether the particular directory addresses have any significance (i.e., are valid). For a cache hit to occur, the F/E bit must be set to a binary ONE. A binary ZERO indicates the presence of an empty block or portion thereof. The round robin bits provide a count which indicates which block was replaced last. This count when read out via one of the two sets of AND gates of block 750-504 into a register 750-506, is normally incremented by one by an increment adder circuit 750-508. The resulting signals NXTRR0-RR2 are written into directory 750-500 to identify the next block to be relaced.

As seen from the Figure, the F/E bit contents of the location are read out via the positions of a two position ZFER selector switch 750-506 and applied as inputs to the directory hit/miss and hit control circuits of block 750-512. The ZFER switch 750-506 selects which half of a group of F/E bits are to be used by the circuits of block 750-512 for a hit/miss indication and which half of the group of F/E bits are to be used by such circuits for an alternate hit determination. An address bit signal ZDAD31 controls the selection of switch positions.

The circuits of block 750-510 include a multisection multiplexer circuit which generates the output signals FEDAT0100 and FEDAT1100 as a function of the hit and miss data pattern. Accordingly, these signals are set in response to the ALTHIT signal from the circuits of block 750-512. A pair of decoder circuits 750-520 and 750-521 operate to decode the level information signals ZLEV0100-2100 for generating appropriate sets of write enable strobe signals R/WFE010-210 and R/WLV010-710 for control directory 750-500 and address directory 750-502. Thus, level (ZLEV) switch 750-522 operates to control the level at which F/E bits are set or reset and the level in the address directory 750-502 at which new addresses are written during a directory assignment cycle of operation.

As seen from the Figure, the first position of ZLEV switch 750-522 when selected, applies to its output terminals sinals OLDRR010-210 from directory 750-500. The second position of switch 750-522 when selected applies to its output terminals signals RLEVR0-R2 from a level register 750-524. The level register 750-524 is used to save the last set of hit level signals generated by the hit/miss level network circuits of block 750-512. This permits the hit level value to distribute to other sections of cache 750 for subsequent use (i.e., signals RHITLEV0-2).

The third position of switch 750-522 when selected applies to its output terminals, signals LEVR0-R2 generated by the circuits of block 750-512. The switch 750-522 is controlled by signals from conrol flip-flops included within block 750-526 (i.e., signals FBYPCAC and DIRBUSY). As seen from the Figure, the complements of the level signals stored in register 750-524 corresponding to signals RHITLEV010-210 are applied via group of AND gates to control circuits within section 750-9.

During the search cycle of operation, the hit/miss level network circuits detect which level, if any, contains an address which matches the ZAC address. In the case of match, if forces signal RAWHIT100 to a binary ONE and generates therefrom the sets of hit level signals ZCD010-210 and HITLEVC7010-7210 through an encoding circuit. The signals are generated in accordance with the states of the F/E bit signals ZFE010-710. That is, for a cache hit to occur at a given level, the F/E bit must be a binary ONE. As mentioned above, a binary ZERO indicates the presence of an empty block level. Each encoder circuit includes AND/OR gating circuits, conventional in design which generate the level signals in accordance with the Boolean expression L ##EQU1## ZHTj.ZFEj. Additionally, the signals ZCD010-210 also may be generated from the level signals ZNICLEV000-2100 provided by section 750-9 during instruction fetches.

The block 750-512 also includes an alternate hit network which can also be used in the assignment of an eight word block by generating an alternate hit signal ALTHIT100 and a set of signals ALTHITLEV0100-2100 for loading into register 750-504 in place of the round robin assignment signals C7RR0100-2100. For the purpose of the present invention, such arrangements can be considered conventional in design. Reference may be made to U.S. Pat. No. 3,820,078 listed in the introductory portion of this application.

As seen from the Figure, the circuits of block 750-512 generate other hit signals HITTOTB100, HITTOC7100 and HITTOIC100. These signals are derived from signal RAWHIT100 in accordance with the following Boolean expressions:

1. HITTOC7100=RAWHIT100.BYPCAC000.

2. HITTOIC100=HITTOC7100.

3. HITTOTB100=RAWHIT100.BYPCAC000+PRERD100.BYPCAC100.

The circuits of block 750-512 receive the cache bypass signals BYPCAC000 and BYPCAC100 from block 750-526. As mentioned, this block includes a number of control state flip-flops which generate signals for sequencing the section 750-5 through various required operations for the processing of the various types of commands. Additionally, block 750-512 includes logic circuits for generating required control signals during such operations. For the purpose of the present invention, these circuits may be implemented in a conventional manner. Therefore, in order to simplify the description herein, only a brief description and the Boolean expressions will be given for certain control state flip-flops and control logic circuits as required for an understanding of the operation of the present invention.

CONTROL STATE FLIP-FLOPS

The FJAM1 flip-flop is set in response to a hit condition at the end of a directory search cycle for a read double command. The flip-flop holds the lower address bits in register(s) 750-32 enabling the accessing of the second word from cache storage unit 750-300 in the case of a read double command. Also, the flip-flop is set in response to a write single command to cause the selection of the RDAD position of the ZDAD switch 750-530 for providing or causing the same address to be applied to cache storage unit 750-500 for one more clock interval or cycle. In the absence of a hold condition (signal [HOLDDMEM=1), the FJAM1 flip-flop remains set for one cycle in accordance with the following Boolean expression:

SET=FJAM1=REQCOMB.RAWHIT.BYPCAC.(RDDBL+WRTSNG)+HOLDDMEM.FJAM2+HOLDDMEN.FJAM

The FJAM2 flip-flop is set in respone to a hit condition at the end of a directory search cycle for a write double command. The setting of the FJAM2 flip-flop causes the setting of the FJAM1 flip-flop at the end of the next clock interval. The control state of the FJAM2 flip-flop together with the FJAM1 flip-flop causes the selection of the RDAD position of ZDAD switch 750-530 for providing the proper address for writing data into cache storage unit 750-300.

The FJAM2 flip-flop also remains set for one cycle in accordance with the following Boolean expression:

SET=FJAM2=REQCOMBO.RAWHIT.BYPCAC.WRTDBL+HOLDDMEM.FJAM2.

A flip-flop NRMPTC1 directly controls the ZDAD switch 750-520 and is set in accordance with the states of signals generated by the other control state flip-flops.

The NRMPTC1 flip-flop normally remains set for one cycle in accordance with the following Boolean expression:

SET=NRMPTC1=(WRTDBL.REQCOMBO.RAWHIT.BYPCAC)+FJAM2+SETFJAM1+REQCOMBO.(RDTYPE.BYPCAC+RDTYP.RAWHIT).(FJAM1.FJAM2+HOLD).

The FDIRASN flip-flop specifies a directory assignment cycle of operation wherein an associative address entry is written into address directory 750-500 in the case of miss conditions or cache bypass operations for read type commands.

The FDIRASN flip-flop is set for one cycle in accordance with the following Boolean expression: SET=FDIRASN=REQCOMBO.RDTYP.(BYPCAC+RAWHIT).

The FICENAB flip-flop enables the loading of the instruction register and is set for one cycle in response to a 1/2 T clock pulse in accordance with the following Boolean expression.

SET=FHT100.

The FRCIC flip-flop is set for one cycle in response to a 1/2 T clock pulse in accordance with the following Boolean expression.

SET=FJAMZNICLEV.

CONTROL LOCIG SIGNALS

1. The ADTHIT signal indicates the presence of a hit condition.

ALTHIT=ALTLEV0+ALTVEV1+. . . ALTLEV7.

2. The signals ALTHITLEV0, ALTHITLEV1 and ALTHITLEV2 provide a three bit code which specifies the level at which a pseudo hit condition occurred. The signals are coded as follows:

a. ALTHITLEV0=ALTLEV4+ALTLEV5+ALTLEV6+ALTLEV7.

b. ALTHITLEV1=ALTLEV2+ALTLEV3+ALTLEV6+ALTLEV7.

c. ALTHITLEV2=ALTLEV1+ALTLEV3+ALTLEV5+ALTLEV7.

3. The signals ALTLEV0 through ALTLEV7 indicate which one of the eight levels, if any, has detected a pseudo hit condition.

a. ALTLEV0=ZHT0.ZFE0.

b. ALTLEV7=ZHT7.ZFE7.

4. The DIRADDE signal is an enabling signal for decoder 750-521 which allows the generation of write strobe signals applied to address directory 750-500.

DIRADDE=NOGO.FDIRASN.

5. The DIRBUSY signal indicates when the directories 750-500 and 750-502 are busy.

DIRBUSY=FLSH+FJAM2+FJAM1+FDIRASN.

6. The FEDCODE signal is an enabling signal for decoder 750-520 which allows the generation of write strobe signals applied to control directory 750-500.

FEDCODE=FDIRASN.NOGO.

7. The FORCEBYP signal enables a cache bypass operation to take place.

FORCEBYP=FSKIPRR+FBYPCAC.

8. The GSRCH signal indicating when a search cycle of operation is to take place.

GSRCH=RDDBLZCDE.FICENAB.FRCIC.

9. The signals HITLEVC70, HITLEVC71 and HITLEVC72 provide a 3-bit code which specifies the level at which a hit condition has occurred.

a. HITLEVC70=HITLEV4+ITLEV5+HITLEV6+HITLEV7.

b. HITLEVC71=HITLEV2+HITLEV3+HITLEV6+HITLEV7.

c. HITLEVC72=HITLEV1+HITLEV3+HITLEV5+HITLEV7.

10. Signals HITLEV0 through HITLEV7 indicate which one of the eight levels, if any, has detected a hit condition.

a. HITLEV0=ZFE0.ZHT0.

b. HITLEV7=ZFE7.ZHT7.

11. The RAWHIT signal indicates the detection of a hit condition.

RAWHIT=HITLEV0+ . . . +HITLEV7.

12. The HITTOC7 and HITTOIC signals each indicates the detection of a hit condition to certain circuits within section 750-9.

HITTOC7=HITTOIC=RAWHIT.BYPCAC.

13. The HITTOTB signal indicates the detection of a hit condition or a pre-read command when in the bypass mode to the transit block buffer circuits.

HITTOTB=RAWHIT.BYPCAC+PRERD.BYPCAC.

14. The LDRAD signal enables the loading of the RDAD register 750-532.

LDRDAD=HOLDDMEM.

15. The LDRDRIN signal enables the loading of RDRIN register 750-534.

LDRDRIN=FDISASN.

16. The signal RDDBLZCDE is used to enable the ZCD switch 750-306 in the case of a read double command.

RDDBLZCDE=FICENAB.(FDIRASN+FJAM1+FJAM2).

17. The REQCOMBO signal indicates the presence of a cache request.

REQCOMBO=NOGO.HOLDDMEM.[CANCELC.DREQCAC.

18. The ZCD0, ZCD1 and ZCD2 signals are used to control the operation of the ZCD switch 750-306.

a. ZCD0=ZCDL4+ZCDL5+ZCDL6+ZCDL7+ZNICLEV0.ZCDICENAB+RDDBLLO.

b. ZCD1=ZCDL2+ZCDL3+ZCDL6+ZCDL7+ZNICLEV1.ZCDICENAB+RDDBLL1.

c. ZCD2+ZCDL1+ZCDL3+SCDL5+SCDL7+ZNICLEV2.ZCDICENAB+RDDBLL2

wherein the term(s) ZCDLi is ZCDLEVi.

19. The ZFEDATWT1 signal is a data write strobe signal used for writing F/E bit signals FEDAT0100 and FEDAT1100 into directory 750-500.

ZFEDATWT1=FDIRASN.ZDAD31.

20. The FEDAT0100 signal corresponds to the first full/empty bit.

FEDAT0100=FBYPCAC000+FALTHIT100.

21. The FEDAT1100 signal corresponds to the second full/empty bit.

FEDAT1100=FALTHIT100+FBYPCAC000.

22. The SELZDADC1 signal controls the operation of the ZDAD switch 750-530.

SELZDADC1=NRMPTC1.

23. The RWRR signal is a round robin write signal used for writing the RR bit signals back into directory 750-500.

RWRR=FDIRASN.NOGO.$CLOCK.

It will be seen from the Figure that the different decoded command signals are generated by a decoder circuit 750-528 in response to the signals applied to the DMEM lines 0-3 by processor 700. The decoder 750-528 is enabled by a signal from the DREQCAC line. The decoded command signals (e.g. WRTDBL, WRTSNG, PRERD, RDTYPE) together with other control signals such as [HOLDDMEM, FSKIPRR00 and those from the lines [CANCELC and BYPCAC are applied as inputs to the circuits of block 750-526.

Instruction Buffer Section 750-7

This section receives memory data and instructions from the DFS lines which are transferred to processor 700 via the ZDI switch 750-312 and ZIB switch 750-314 respectively. The memory signals are loaded into an RDFS register 750-702 via one position of a two position switch 750-700.

Memory data fetched as a result of a miss condition upon receipt is applied to the ZDI switch 750-312 via the RDFS position #0 of a 1 of 4 position (ZDIN) switch 750-708. In the case of a load quad command, memory data is loaded into the 4 location (LQBUF) buffer 750-706 when the [LQBUF signal is forced to a binary logical ONE. The write/read address signals [WRTBUF010110/[RDBUF010-110 from section 750-112 control the writing and reading of data into and from the locations of buffer 750-706.

The memory data stored in the LQBUF buffer 750-706 is then transferred to the ZDI via the RLQBUF position #2 of the ZDIN switch 750-708.

In the case of a read double command, the even word of the pair is transferred into a REVN register 750-710. Thereafter, the even word is transferred to the ZDI switch 750-312 via position #1 of ZDIN switch 750-708 for execution of a read double odd command request or upon receipt of a RD-EVEN signal from processor 700.

As seen from the Figure, each memory data word is also loaded into the RDFSB register 750-712 and thereafter written into cache storage unit 750-300 via the ZCDIN switch 750-304 at the level specified by the contents of the RADR register 750-32.

In the case of instruction transfers, each instruction received from memory is loaded into one of the 4 storage locations of a specified one (IBUF1/IBUF2) of a pair of instruction buffers 750-715 and 750-717. The IBUF1 and IBUF2 buffers 750-715 and 750-717 are used to buffer up to two four word blocks that can be accessed from memory in response to I fetch 1 or I fetch 2 commands for which a miss condition has been detected.

The instructions are written into the location of one of the IBUF1 and IBUF2 buffers 750-715 and 750-717 specified by signals [WRTBUF0100-1100 under the control of write strobe signals [IBUF1/[IBUF2. Read control signals [RDBUF0100-1100 enable the read out of such instructions for transfer to processor 700 whenever the IBUF1 or IBUF2 location specified by the signals [ZEXT0100-1100 contains an instruction. The instruction is transferred to processor 700 via positions 1 or 2 of a two position switch 750-720 and the ZRIB switch position of the ZIB switch 750-314.

The IBUF1 and IBUF2 buffers 750-715 and 750-717 apply output valid signals IBUF1V100 and IBUF2V100 to IBUFREADY circuits of block 750-722. These circuits force IBUFRDY line to a binary ONE indicating that there is at least one instruction in the I buffer being addressed (current instruction block). As seen from the Figure, the IBUFREADY circuits receive input signals (e.g. USETBRDY, IFETCHRDY) from control circuits within section 750-9.

Instruction Counter Section 750-9

This section stores cache address signals (24-33) for indicating the next instruction to be accessed, in one of two instruction address registers (RICA/RICB) 75-900 and 750-902. The cache address signals 24-33 are loaded into the instruction register RICA/RICB not being used when an IFETCH1 command is received from processor 700. The cache address is transferred via the RADO position of ZDAD switch 750-530 and a ZDAD position #0 of a 4 position ZICIN switch 750-904.

Each time processor 700 accesses an instruction, the contents of the instruction register RICA/RICB read out via one position of a two position ZIC switch 750-906 are incremented by one via an increment circuit 750-908. The incremented contents are returned to the instruction register RICA/RICB via the RNIC position #1 of ZICIN switch 750-904.

As seen from the Figure, each instruction register stores two level fields for fetching first and second blocks of instructions in response to IFETCH1 and IFETCH2 commands. The two pairs of level field signals are applied to the different switch positions of a 4 position crossbar switch 750-910. The selected level signals ZNICLEV0100-2100 applied as inputs to block 750-512 are used to control the operation of ZCD switch 750-306 for accessing the instructions specified by the instruction register RICA/RICB. The level field signals correspond to signals HITLEVC70100-2100 which are generated by the circuit of block 750-512. These signals are loaded into one of the instruction registers following a directory assignment cycle of operation.

In addition to the level field signals, the RICA and RICB instruction address registers store other signals used for various control purposes which will be discussed herein to the extent necessary.

The incoming cache address signals from the ZDAD switch 750-530 are incremented by one via another increment circuit 750-912. The incremented address signals are loaded into the RICA/RICB instruction register via the INC position #3 of ZICIN switch 750-904. The least significant two bits 32-33 of the cache address provide the IBUF1 or IBUF2 address (i.e., signals ZEXT0100-1100) to read out instruction blocks accessed from memory.

It will be noted that the pair of level field signals LEV1 and LEV2 from other outputs of switch 750-910 are applied as inputs to a pair of comparator circuits 750-912 and 750-914. The circuits 750-912 and 750-914 compare the level signals LEV1 and LEV2 of the current instruction block from switch 750-910 with the input level signals C7RR0100-2100 corresponding to the round robin count for the next available block. Also, the comparator circuit 750-912 receives as inputs memory level signals RTBLEV0100-2100 and instruction level signals ZNICLEV0100-2100 from switch 750-910 for comparison in addition to level signals ZIC0100-2100 for comparison with signals C7RR0100-2100. The cache address signals are incremented by 4 by an increment circuit 750-918 and applied as an input to the round robin skip control circuits of block 750-916. These circuits receive as another pair of inputs the input cache address signals 24-30 from ZDAD switch 750-530 and the cache address signals of the current instruction block from ZIC switch 750-906 for comparison by circuits included therein.

The results of the pairs of cache address signals and level signal comparison are combined within other circuits within the round robin skip control circuits of block 750-916. The circuits of block 750-916, in response to decoded signals from a decoder circuit 750-922, generate output control signals which avoid addressing conflicts. For a further discussion of the operation of such circuits, reference may be made to the copending application of Marion G. Porter, et al titled "Cache Unit Information Replacement Apparatus" referenced in the introduction of this application.

The output control signals from block 750-916 are applied as inputs to the circuits of IC control block 750-920. Additionally, the control circuits of block 750-920 receive the results of the decoding of command signals applied to the DMEM lines by the decoder circuit 750-922 when it is enabled by a signal from the DREQCAC line. These decoded signals together with the other signals from sections 750-1 and 750-5 are applied to block 750-920. The control circuits of block 750-920 generate address and control signals for sequencing section 750-9 through the required cycles of operation for processing certain types of commands (e.g. IFETCH1, IFETCH2 and LDQUAD commands).

The block 750-920 includes a number of control state flip-flops and logic circuits for generating the required control signals. For the same reasons mentioned in connection with section 750-5, only a brief description and the Boolean expressions will be given for certain state flip-flops and control circuits.

CONTROL STATE FLIP-FLOPS

FABCURLEV1 flip-flop defines the current level for the RICA/RICB instruction register. This flip-flop is set and reset in response to a T clock timing signal in accordance with the following Boolean expressions. The set condition overrides the reset condition. When FA/FBCURLEV is a binary ZERO, it selects level 1 and when a binary ONE, it selects level 2.

SET=DECODEIF1.F-PPIMEIS.[HOLDDMEN.[CANCELC.ZDAD08.ZDAD09.HIT.FACTVRIC100/000+ZEXT0.ZEXT1.RDIBUF.HOLDEXECRDIBUF.FA/FBCURLEV000.DECODELDQUAD.FLDQUAD.DECODEEIS.FACTVRIC100/000.NOGO+ZEXT0.ZEXT1.FLDQUAD.RDIBUF.HOLDEXECRDIBUF .FACTVRIC100/000.NOGO.

RESET=DECODEIF1.FFPIMEIS.[HOLDDMEM.[CANCELC.FACTVRIC100/000+DECODELDQUAD.[HOLDDMEM. [CANCELC.FACTVRIC100/000+ZEXT0.ZEXT1.DECODELDQUAD.FLDQUAD.DECODEIF1.FA/PBCMPLEV100.FACTVRIC000/100.RDIFUB.HOLDEXECRDIBUF.NOGO.

The FACTVRIC flip-flop specifies the currently active instruction register RICA/RICB. When the flipflop is set to a binary ONE, it specifies the RICA register and when a binary ZERO, it specifies the RICB register. It is set and reset in response to a T clock timing pulse signal in accordance with the following Boolean expressions.

FACTVRIC=FACTVRIC.TGLACTVRIC

wherein TGLACTVRIC=DECODEIF1.[HOLDDMEM.[CANCELC.FFPIMEIS+FNEWIF1.NOGO.

FACTVRIC=FACTVRIC.TGLACTVRIC wherein

TGLACTVRIC=(DECODEIF1+[HOLDDMEM+[CANCELC+FFPIMEIS).(FNEWIF1+NOGO).

The FCPUWRTREQ flip-flop defines the time during which processor data is to be written into cache. It is set and reset in response to a T clock timing pulse in accordance with the following Boolean expressions.

SET=(DECODEWRTSNGL+DECODEWRTDBL).HIT.[HOLDDMEM.[CANCELC.

RESET=FWRTDBL.HOLDCACHECPUWRTSEQ.

The FDBLMISS flip-flop defines a read double type miss condition and is used to select the ZDIN position of ZDI switch 750-312 during the cycle following data recovery. It is set and reset in response to a T clock timing pulse in accordance with the following Boolean expressions.

SET=(DECODERDDBL+DECODERDRMT).[HOLDDMEM.[CANCELC.MISS.

RESET=FRDMISS.

The FEVENODD flip-flop specifies which word of the two word pairs processor 700 is waiting for when a read single type miss condition occurs. The flip-flop also defines the order that the data words are to be returned to processor 700 in the case of a read double type miss condition.

Further, the flip-flop is used during a read double hit condition to access the second data word. It is set and reset in response to a T clock timing pulse in accordance with the following Boolean expressions.

SET=(DECODERDSCNGL+DECODEIF1.FFPIMEIS).[HOLDDMEM.[CANCELC.ZDAD09+DECODERDDBL.[HOLDDMEM.[CANCELC.DSZ1.

RESET=(DECODERDSNGL+DECODEIF1).[HOLDDMEM.[CANCELC.ZDAD09+DECODERDDBL.[HOLDDMEM.[CANCELC.DSZ1+DECODERDRMT.[HOLDDMEM.[CANCELC.

The FFPIMEIS flip-flop specifies that the last processor state was an FPIMEIS state which means that the IF1 command on the DMEM lines is a request for addition EIS descriptors. This flip-flop is set and reset in response to a T clock pulse in accordance with the following Boolean expressions.

SET=FPIMEIS.

RESET=DECODEIF1.[CANCELC.[HOLDDMEM.

The FHOLDIF1 flip-flop defines when processor 700 is being held because of an IF1 miss condition so that when the instruction is received from memory, the current instruction register RICA/RICB can be updated by the FDATARECOV flip-flop. The flip-flop is set and reset in response to a T clock pulse in accordance with the following Boolean expressions.

SET=DECODEIF1.IFPIMEIS.[HOLDMEM.[CANCELC.MISS.

RESET=FNEWIF1.NOGO+FDATARECOV.

The FINHRDY flip-flop is used to inhibit the signaling of an IBUFRDY condition to processor 700 when a conflict occurs between the instruction (IC) level and memory data level at the time processor 700 took the instruction loaded into RIRA/RIRB from cache. It is set in response to a T clock pulse and is reset unconditionally on the next T clock pulse when no set condition is present. It is set in accordance with the following Boolean expression.

SET=SETIRTERM.READIBUF.[HOLDDMEM.NOGO.

wherein SETIRTERM=CMPDATAICLEV+MEMWRTREQ.(ZEXT0.ZEXT1.IF2.[CANCELCMD+DECODEIF1.FFPIMEIS+FINHRDY).

RESET=SET.

The FJAMZNICLEV flip-flop is used to force the level signals ZNICLEV000-2100 of the next instruction to be applied to the control input terminals of ZCD switch 750-306 (i.e., signals ZCD010-210) following an IF1 command which did not specify the last word in the block. The flip-flop is set in response to a T clock pulse in accordance with the following Boolean expression. It is reset on the occurrence of the next T clock pulse.

SET=DECODEIF1.FFPIMEIS.HIT.[HOLDDMEM.[CANCELC.[CANCELC.(ZDAD08.ZDAD09).

The FNEWIF1 flip-flop defines the cycle after an IF1 command is received from processor 700. It is set for one cycle in response to a T clock pulse in accordance with the following Boolean expression.

SET=DECODEIF1.FFPIMEIS.[HOLDDMEM.[CANCELC.

The FRDIBUF flip-flop is used to specify that a signal on the RDIBUF line was received from processor 700 during the last cycle of operation. It is set in accordance with the following Boolean expression. It is reset during the next cycle in the absence of a set condition.

SET=RDIBUF.HOLDEXECRDIBUF.NOGO.

The FRDMISS flip-flop is used to cause the holding of processor 700 upon detecting a miss condition for any read type command. It is set and reset in response to a T clock pulse in accordance with the following Boolean expressions.

SET=(DECODERDSNGL+(DECODEIF1.FFPIMEIS)+DECODERDRMT+DECODERDCLR+DECODERDDBL).[HOLDDMEM.[CANCELC.MISS.

RESET=FDATARECOV+FNEWIF1.NOGO.

The FRDREQ flip-flop defines when the second word fetched in response to a RDDBL command for a hit condition is to be read out from cache. It is set and reset in response to a T clock pulse in accordance with the following Boolean expressions.

SET=DECODERDDBL.HIT.[HOLDDMEM.[CANCELC.

RESET=[HOLDDMEM.

The FDATARECOV flip-flop inhibits the incrementing of the instruction register RICA/RICB when the IF1 command is to the last word in the block and the IF2 command is cancelled. It is set and reset in response to a T clock pulse in accordance with the following Boolean expressions:

SET=DATARECOV.FLASTINST.[HOLDDMEM.[CANCELC+DATARECOV .FLASTINST.[CANCELC.[HOLDDMEM+DATARECOV. FLASTINST.

RESET=[HOLDDMEM.FDATARECOV.

CONTROL LOGIC SIGNALS

1. The FA/FBLEV1VAL signal is used to define the state of a first valid bit position of the RICA/RICB instruction register. It is set and reset on a T clock pulse in accordance with the following Boolean expressions. The reset condition overrides the set condition.

a. FA/FBLEV1VALSET=DECODEIF1.FFPIMEIS.[HOLDDMEM.[CANCELC.FACTVRIC100/000+DECODEIF1.FFPIMEIS.[HOLDDMEM.[CANCELC.EISIF2.FACTVRIC000/100+DECODELDQUAD.[HOLDDMEM [CANCELC FACTVRIC100/000.

b. FA/FBLEVIVAIRESET=DECODEIF1.FFPIMEIS.[HOLDDMEM.[CANCELC.HIT.ZDAD08.ZDAD09.FACTVRIC100/000+ZEXT0.ZEXT1.DECODEIF1.DECODELDQUAD.FLDQUAD.RDIBUF.HOLDEXECRDIBUF.FACTVRIC000/100.FA/FBCMPLEV000.NOGO+ZEXT0.ZEXT1.FLDQUAD.RDIBUF.HOLDEXECRDIBUF.FACTVRIC100/000 NOGO.

wherein RICA=FACTVRIC=1 and RICB=FACTVRIC=1.

2. The FA/FBLEV2VAL signal is used to define the state of a second valid bit position of the RICA/RICB instruction register. It is set and reset on a T clock pulse in accordance with the following Boolean expressions.

a. FA/FBLEV2VALSET=DECODEIF2.[HOLDDMEM.[CANCELC.FACTVRIC000/100.NOGO+DECODEIF1.FFPIMEIS.[HOLDDMEM.[CANCELC.FACTVRIC000/100.EISIF2.

b. FA/FBLEV2VALRESET=DECODEIF1.FFPIMEIS.[HOLDDMEM.[CANCELC.FACTVRIC100/000+DECODELDQUAD.[HOLDDMEM.[CANCELC.FACTVRIC100/000+ZEXT0.ZEXT1.DECODEIF1.DECODELDQUAD.FLDQUAD.FA/FBCURLEV.FACTVRIC000/100.RDIBUF.HOLDEXECRDIBUF.NOGO. wherein RICA=FACTVRIC=1 and RICB=FACTVRIC=1.

3. The [ZIB0 and [ZIB1 signals control the ZIB switch for transfers of instructions from cache 750 to processor 700 via the ZIB lines.

a. [ZIB0=IFETCHRDY.FNEWIF1.

B. [ZIB1=IFETCHRDY.

4. The [ZDI0, [ZDI1 and [ZDI2 signals control the ZDI switch for transfers of instructions and data from cache 750 to processor 700 via the ZDI lines. Control signal [ZDI9, which corresponds to the most significant bit of the three bit code, can be assigned to be a binary ZERO unless positions 4 through 7 are being used for display purposes.

a. [ZDI1=DATARECOV+BDBLMISS+RDEVEN.

b. [ZDI2=RDIBUF/ZDI.(HITTOIC+FRDREQ).

5. The [ZICIN0 and [ZICIN1 signals control the ZICIN switch for loading address signals into the RICA and RICB instruction address registers 750-900 and 750-902.

a. [ZICIN0+ALTCMD100.FDFN2HT.[HOLDDMEM.

b. [ZICIN1=FDFN1HT.FNEWIF1+FDFN2HT.

6. The signals ENABRIC1 and ENABRIC2 are used to enable the loading RICA and RICB registers.

a. ENABRIC1=FHOLDIF1.FNEWIF1.FJAMZNICLEV.[HOLDDMEM.FDATARECOV+FHOLDIF1.DATARECOV.

b. ENABRIC2=FINHRDY.SETINHRDY.DFN2HT wherein SETINHRDY=DFN2T.[MEMWRTREQ (ZEXT0.ZEXT1.EXECIF2.[CANCLCMD+FINHRDY+PSUEDOIF1+PSUEDOIF2)+CMDATA/ICLEV].

7. The signal DATARECOV defines the time that new data has been loaded into the processor's registers (e.g. RDI or RBIR) and when the processor is released. This signal is generated by a flip-flop of section 750-1 which is set to a binary ONE in response to a T clock pulse upon detecting an identical comparison between the address signals specifying the word requested to be accessed by processor 700 and signals indicating the word being transferred to cache unit 750. The comparison indicates that signals DATA, MIFS2, MIFS3, MIFS1 and DATAODD are identical to signals FHT, FHOLDTB0, FHLDTB1, RADR32 and DOUBLEODD respectively

wherein

signal FHOLDTB0=FRDMISS.LDTBVALID.FIF2ASSIGN.FTBPTR0;

signal FHOLDTB1=FRDMISS.LDTBVALID.FIF2ASSIGN.FTBPTR1;

signal DOUBLEODD=FEVENODD.FDPFS; and

signal DATA=FARDA+FDPSF.

DETAILED DESCRIPTION OF SECTION 750-1

FIG. 7a shows in greater detail different ones of the blocks of section 750-1. It will be noted that for the purpose of facilitating understanding of the present invention, the same reference numbers have been used to the extent possible for corresponding elements in FIG. 4. In many cases, a single block depicted in FIG. 4 includes several groupings of circuits for controlling the operation thereof and/or for generating associated control signals. Therefore, some blocks with appropriate reference numbers are included as part of the different blocks of section 750-1.

Referring to the Figure, it is seen that certain portions of block 750-102 are shown in greater detail. The transit block buffer 750-102 is shown as including a first group of circuits for keeping track of data words received from memory in response to a read quad type command. These circuits include a plurality of clocked pair count flip-flops which comprise a four-bit position register 750-10200, a multiplexer circuit 750-10202, a plurality of NAND gates 750-10204 through 750-10210 and a decoder circuit 750-10212. It will be noted that there is a pair count flip-flop for each transit buffer location.

Additionally, the first group of circuits includes a plurality of clocked transit block valid flip-flops which comprise a four-bit position register 750-10214. The binary ONE outputs of each of the flip-flops are connected to a corresponding one of the four pair count flip-flops as shown.

In response to a read quad command, a first pair of words is sent to cache 750. This is followed by a gap and then the second pair is sent to cache 750. The pair count flip-flop associated with the transit block buffer location being referenced as specified by the states of signals MIFS2110 and MIFS3110 is switched to a binary ONE via a first AND gate in response to T clock signal [CLKT022 when signal DATAODD100 is forced to a binary ONE by the circuits of block 750-114. Signal RESETTBV100 is initially a binary ZERO and decoder circuit 750-10212 operates to force one of the first four output signals SETPC0100 through SETPC3100 in accordance with the states of the MIFS2110 and MIFS3110 from switch 750-128.

The pair count flip-flop is held in a binary ONE state via the other input AND gate by a transit block valid signal associated therwith being forced to a binary ONE. The appropriate one of the transit block valid bit flip-flops designated by decoder circuit 750-10601 (i.e., signals IN0100 through IN3100) is set to a binary ONE via a first AND gate, when switching takes place, increment signal INCTBIN100 is forced to a binary ONE state in response to T clock signal [CLKT022.

The multiplexer circuit 750-10202 in accordance with the states of the signals DMIFS2100 and DMIFS3100 from switch 750-128 selects the appropriate binary ONE out of the four pair count flip-flops to be applied to NAND gate 750-10204. This causes NAND gate 750-10204 to force signal LASTODD100 to a binary ZERO. This results in NAND gate 750-10206 forcing signal LASTDTAODD000 to a binary ONE.

When the next pair of data words are received, this causes NAND gate 750-10206 to force signal LASTDTAODD000 to a binary ZERO. This, in turn, causes NAND gate 750-10210 to force reset signal RESETTBV1100 to a binary ONE. The decoder circuit 750-10212 is conditioned by signal RESETTBV100 to force one of the four output terminals 4 through 7 to a binary ONE. This, in turn, resets the appropriate one of the transit block valid bit flip-flops via the other AND gate. As soon as the TB valid flip-flop resets, it resets the pair count flip-flop associated therewith via its other AND gate. It will be appreciated that such switching occurs in response to T clock signal [CLKT022.

As seen from FIG. 7a, the first group of circuits of block 750-102 further includes a plurality of NAND gates 750-10216 through 750-10222, each of which is connected to receive a different one of the binary ONE outputs from register 750-10214. The binary ONE outputs FTBV0100 through FTBV3100 are also connected to the control input terminals of the transit block address comparator circuits 750-132 through 750-136.

Each of the NAND gates 750-10216 through 750-10222 also is connected to receive a different one of the signals IN0100 through IN3100 from decoder circuit 750-10601. The outputs from these gates are applied to an AND gate 750-10224. The signals VALID000 through VALID3000 are used to indicate when a transit block register location is available for writing. That is, when a selected transit block valid bit flip-flop is in a reset state, AND gate 750-10224 maintains signal VALIDIN000 in a binary ONE state.

The VALIDIN000 signal conditions a further AND/NAND gate 750-10226 to force a control signal [RTB100 to a binary ONE during the second half of a cycle of operation (i.e., signal FHT020 is a binary ONE) in the case of a read command (i.e., signal DREQREAD100 is a binary ONE) at the time a directory assignment is not being made (i.e., signal FLDTBVALID000 is a binary ONE).

As seen from FIG. 7a, control signal [RTB100 is applied via a driver circuit 750-10228 to a decoder circuit 750-10230. The control signal [RTB110 causes the decoder circuit 750-10230 to force an appropriate one of the output signals [RTB0100 through [RTB3100 designated by the states of signals FTBPTR0100 and FTBPTR1100 applied via a pair of driver circuits 750-10232 and 750-10234 to a binary ONE state. This in turn causes bit positions 24-31 of one of the transit block register locations to be loaded with address signals applied via the RADO lines 24-31. The complement signal [RTB000 is applied an an input to block 750-107 for controlling the loading of command queue 750-107.

A second group of circuits of block 750-102 shown in greater detail includes the transit block buffer flag storage section 750-10238 of buffer 750-102. This section as well as the section of buffer 750-102, not shown, is constructed from a 4×4 simultaneous dual read/write memory. The memory is a 16-bit memory organized as 4 words of 4 bits each, only three bits of which are shown. Words may be independently read from any two locations at the same time as information is being written into any location. The signals FTBPTR0100 and FTBPTR1100 are applied to the write address terminals while the read addresses are enabled by the VCC signal applied to the G1 and G2 terminals. The Y bit locations are selected in accordance with the states of read address signals MIFS3100 and MIFS2100 from switch 750-128. The Z bit locations are selected in accordance with the states of signals DMIF3100 and DMIF2100 from switch 750-128. Since these locations are not pertinent they will not be discussed further herein.

The memory may be considered conventional in design, for example, it may take the form of the circuits disclosed in U.S. Pat. No. 4,070,657 which is assigned to the same assignee as named herein. Upon the receipt of memory data, the flag bit contents of the transit block location specified by signals MIFS2100 and MIFS3100 are applied to the Y output terminals. These signals are in turn applied to blocks 750-102, 750-115 and 750-117, as shown. During the directory assignment cycle for a cache read miss, the flag bit positions of the transit block location specified by signals FTBPTR0100 and FTBPTR1100 are loaded with the signals FORCEBYP000, FRDQUAD100 and FLDQUAD100 generated by the circuits of blocks 750-5 and 750-114.

It is also seen from FIG. 7a that block 750-102 further includes a group of instruction fetch flag circuits which are associated with the operation of transit block buffer 750-102. These circuits include two sets of input AND gates 750-10240 through 750-10243 and 750-10250 through 750-10253, a pair of multiplexer selector circuits 750-10255 and 750-10256, and IF1 and IF2 flag storage register 750-10258 and an output multiplexer circuit 750-10260 arranged as shown.

The binary ONE outputs of the individual IF1 and IF2 flip-flops are connected to corresponding ones of the sets of AND gates 750-10240 through 750-10243 and 750-10250 through 750-10253. These AND gates also receive input signals from the circuits of block 750-106 generated in response to the in pointer signals FTBPTR0000 and FTBPTR1000 used for addressing the different register locations within the buffer 750-102 as mentioned previously.

The multiplexer circuit 750-10255 is connected to receive as a control input, signal FIF1ASSIGN100 from FIF1ASSIGN flip-flop 750-11418. The multiplexer circircuit 750-10256 is connected to receive as a control input signal FIF2ASSIGN100 from FIF2ASSIGN flip-flop 750-1410. This enables the setting and/or resetting of the IF1 and IF2 flip-flops of register 750-10258 in response to the signals FIF1ASSIGN100 and FIF2ASSIGN100. The switching occurs in response to T clock signal [CLKT022 during the loading of a transit block register location when a control signal LDTBVALID100 is switched to a binary ONE via an AND gate 750-11428.

It will be noted that register 750-10258 contains an IF1 and IF2 flag bit position for each transit block register location. That is, the register includes flip-flops FIF10, FIF20 through FIF13, FIF23 for transit block register locations 0 through 3 respectively. Each of the binary ONE outputs from the IF1 and IF2 flag flip-flops are also applied to the different input terminals of the output multiplexer circuit 750-10260. The circuit 750-11450 contains two sections. This permits DMIFS2100 and DMIFS3100 signals applied to the control terminals of the multiplexer circuit 750-10260 from block 750-128 to select as outputs, input signals from both an IF1 and IF2 flag flip-flop. The selected pair of signals, in turn, provide flag signals ZIF1FLG100 and ZIF2FLG100 which are applied to block 750-115. These signals are used to control the writing of memory information into the IBUF1 and IBUF2 buffers 750-715 and 750-717. Additionally, the complements of the outputs from multiplexer circuit 750-10260 which correspond to signals ZIF1FLG000 and ZIF2FLG000 are applied to a pair of input terminals of a multisection comparator circuit 750-110/750-11435.

It will be noted that the last section of each of multiplexer circuits 750-10255 and 750-10256 are connected in series for generating the enable transit block buffer ready signal ENABTBRDY100 applied to block 750-114. As shown, the "0" input terminal of the last section of multiplexer circuit 750-10255 connects to a voltage VCC (representative of a binary ONE) while the "1" input terminal connects to ground (representative of a binary ZERO). The output terminal of the last section of multiplexer circuit 750-10255 connects to the "0" input terminal of the last section of multiplexer circuit 750-10256 while the "1" input terminal connects to ground. The multiplexer circuits 750-10255 and 750-10256 operate to force signal ENABTBRDY100 to a binary ONE only after the completion of an instruction fetch assignment cycle when both signals FIF1ASSIGN100 and FIF2ASSIGN100 are binary ZEROS. Therefore, the "0" input terminals are selected as outputs by the multiplexer circuits 750-10255 and 750-10256 which results in signal ENABTBRDY100 being forced to a binary ONE. This presents the inadvertent generation of the IBUFRDY100 signal as explained herein.

As seen from FIG. 7a, the circuits of the transit buffer in pointer block 750-106 include a clocked two-bit position register 750-10600 and a decoder circuit 750-10601. The register 750-10600 has associated therewith a NAND/AND gate 750-10602 and a two input AND/OR gate 750-10604 connected in a counter arrangement. That is, the NAND gate 750-10602 in response to load signal FLDTBVALID111 from block 750-114 and signal NOGO020 forces an increment signal INCTBIN100 to a binary ONE. This causes the address value stored in register 750-10600 to be incremented by one. The increment signal INCTBIN100 is applied to the circuits of block 750-102.

The most significant high order bit position of register 750-10600 is set to a binary ONE via the gate 750-10604 in response to either signals FTBPTR0100 and FTBPTR1000 or signals FTBPTR1100 and FTBPTR000 being forced to binary ONES. The complemented binary ONE output signals of the register bit positions corresponding to signals FTBPTR0000 and FTBPTR1000 are decoded by decoder circuit 750-10601. The circuit 750-1061 in response to the FTBPTR0000 and FTBPTR1000 signals forces one of the four pairs of output terminals to a binary ONE.

The command control circuit block 750-114 includes an instruction fetch 2 search (FIF2SEARCH) synchronous D type flip-flop 750-11400. The flip-flop 750-11400 is set to a binary ONE state in response to T clock signal [CLKT020 when a two input AND/OR gate 750-11402 and an AND gate 750-11404 force a set signal SETIF2SEARCH-100 to a binary ONE. This occurs when either an IF1 command which is a hit or an IF2 command is received from processor 700 during an IF1 assignment cycle.

In the case of an IF1 command, this presumes that there is no hold condition (i.e., signal [HOLDDMEM000 from block 750-117 is a binary ONE) and that a directory search generated a hit (i.e., signal HITTOTB100 is a binary ONE) indicating that the requested instruction block resides in cache store 750-300. For an IF2 command, it is assumed that there has been a directory assignment cycle following a directory search in which there was a miss made in response to the IF1 command (i.e., signal FIF1ASSIGN100 is a binary ONE).

In either of the situations mentioned, the gate 750-11402 forces the signal SETIF2TIME100 to a binary ONE. When the instruction fetch command was caused by a transfer or branch instruction, which is not a NOGO (i.e., signal NOGO030 is a binary ONE) indicating that it should process the IF2 command currently being applied to the command lines (i.e., indicated by signal DREQCAC112 being forced to a binary ONE), AND gate 750-11404 forces signal SETIF2SEARCH100 to a binary ONE. This switches flip-flop 750-11400 to a binary ONE when signal [CANCEL012 is a binary ONE.

As seen from FIG. 7a, the binary ZERO output from flip-flop 750-11400 is applied as an input to the hold circuits of block 750-117. The signal FIF2SEARCH000 is delayed by a buffer circuit 750-11406 and applied to one input of an input NAND gate 750-11408 of an instruction fetch 2 assignment (IFIF2ASSIGN) flip-flop 750-11410.

The signal FIF2SEARCH010 together with the signal EISIF2000 (indicates a non-EIS type instruction) causes the NAND gate 750-11408 to switch FIF2ASSIGN flip-flop 750-11410 to a binary ONE in response to a gating signal SETBVALID100 and T clock signal [CLKT020. The state of this flip-flop as the others is gated as an output when signal FLDTBVALID111 is a binary ONE.

It will be noted that signal FLTBVALID111 is switched to a binary ONE via an AND gate 750-11412, a clocked flip-flop 750-11414 and a delay buffer circuit 750-11416 in the case of a miss condition (i.e., signal HITTOTB010 is a binary ONE) generated in response to a directory search made for a read type command (e.g. IF2). This assumes that there is no hold condition (i.e., signal [HOLDDMEM000 is a binary ONE), that in the case of an IF2 command it was not due to a transfer NOGO (i.e., signal NOGO020 is a binary ONE) and that there is no cancel condition (i.e., signal [CANCELC010 is a binary ONE) for a read type operation decoded by the circuits of block 750-113 in response to the read command applied to the command lines (i.e., signal DREQREAD100 is a binary ONE wherein DREQREAD100=READ100.DREQCAC112).

Under similar conditions, an instruction fetch 1 assignment (FIF1ASSIGN) flip-flop 750-11418 is switched to a binary ONE via an input AND gate 750-11420 in response to an IF1 command (i.e., when signal IF1100 is a binary ONE) in which there was a miss detected (i.e., signal SETTBVALID100 is a binary ONE). The load transit buffer valid flip-flop 750-11414 remains set until signal SETLDTBVALID100 switches to a binary ZERO. It will be noted that the binary ZERO output signal FLDTBVALID000 is applied to circuits included as part of block 750-102.

The other pair of flip-flops 750-11422 and 750-11424 are set in response to signal SETLDTBVALID100 in the case of a miss condition. The load quad flip-flop 750-11424 is set to a binary ONE state when the command applied to the DMEM command lines is decoded as being a LDQUAD command (i.e., signal LDQUAD100 from decoder 750-113 is a binary ONE) and that the ZAC command applied to the ZADOB lines is coded as requiring a read quad operation (e.g. IF1, IF2, LDQUAD, PRERD and RDSNGLE commands specified by signal ZADOB04100 being set to a binary ONE).

The RDQUAD flip-flop 750-11422 is set to a binary ONE via an AND gate 750-11426 when a signal CQIN1100 from the circuits included within command queue block 750-107 is a binary ONE indicative of a double precision command (i.e., signal ZADOBO2100 is a binary ONE).

As seen from FIG. 7a, block 750-114 further includes a comparator circuit 750-11435. This circuit may be considered conventional in design and, for example, may take the form of the circuits disclosed in U.S. Pat. No. 3,955,177.

The comparator circuit 750-11435 is enabled by signals USETBRDY100 and DATA100. The signal USETBPDY100 indicates that the cache is waiting for instructions from memory to be loaded into the IBUF1 or IBUF2 buffers. The signal DATA100 is forced to a binary ONE by a NAND gate 750-11436 indicating receipt of information from memory. The comparator circuit includes two sections. One section compares the command queue input pointer signals and output pointer signals from blocks 750-108 and 750-109 respectively. This section forces signals CQCMP100 and CQBMP000 to a binary ONE and binary ZERO respectively when the pointer signals are equal. The section corresponds to block 750-110 in FIG. 4.

The other section compares input terminals A1, A2 and B1, B2, the control signals [ZRIB100, [ZRIB010 applied to input terminals A1, A2 to the states of the I fetch 1 and I fetch 2 flag signals ZIF1FLG000, ZIF2FLG000 applied to terminals B1, B2. When equal, this indicates that the information being received from memory at this time is either in response to an I fetch 1 or I fetch 2 command. It will be noted that control signal [ZRIB100 controls ZRIB switch 750-720.

The input terminals A4, A8 compare signals ZEXT0100, ZEXT1100 against signals MIFS1100 and DATAODD100 applied to the B4, B8 terminal. This indicates whether the information being addressed within the instruction buffer equals the information being received. More specifically, signals ZEXT0100 and ZEXT1100 are generated by the circuits of block 750-920 from the least two significant bit address of the instruction stored in the RIRA register. Thus, they specify the word location being addressed within the I buffer. Signal MIFS1100 is coded to specify whether the first or second half of the block is being received. Signal DATAODD100 specifies whether the first or second word of the first two word pairs is being received. The signal DATAODD100 is generated by an AND gate 750-11437.

Lastly, the comparator circuit 750-11435 compares a signal ENABTBRDY100 applied to terminal A16 from block 750-102 with the voltage VCC representative of a binary ONE applied to terminal B16. In the presence of a true comparison between the two sets of all six signals, the circuit 750-11435 forces its output to a binary ONE. This results in the complement output terminal forcing signal IBUFCMPR000 to a binary ZERO. This causes block 750-722 to force the IBUFRDY100 signal to a binary ONE as explained herein.

Additionally, section 750-114 includes an AND gate 750-11417. During the first half of a cache cycle (i.e., signal FHT120 from delay circuit 750-11810 is a binary ONE) when the FLDTBVALID flip-flop 750-11414 is a binary ONE, the AND gate 750-11417 forces control signal [RTB5-8100 to a binary ONE. This signal is applied as a clock strobe input to the level storage section of transit block buffer 750-102. This section is constructed from a 4×4 simultaneous dual read/write 16-bit memory organized as four words each 4 bits in length similar to the memory device of block 750-10238 and the memory devices used in constructing the 36-bit read command buffer section of block 750-102 as well as the write command/data buffer 750-100.

FIG. 7a shows that the data reception and control block 750-115 includes a plurality of NAND gates 750-11500 through 750-11510 and a plurality of AND gates 750-11511 through 750-11514 connected as shown to generate the control strobe enable signals [LQBUF100, [IBUF1100 and [IBUF2100, reset buffer signal RESETBUF100 and write control buffer signal [WRTBUF0100. These signals are used to control the operation of the buffer circuits of section 750-7. As seen from FIG. 7a, the other write control buffer signal [WRTBUF1100 is generated by a buffer delay circuit 750-11515 in response to signal FARDA010. The signal [WRTBUF0100 is derived from the output of the two input data selector/multiplexer circuit 750-128 which selects either the signal RMIFS1100 from register 750-127 or signal RMIFSB1100 from register 750-129. The selection is made in accordance with the state of signal FARDA000 produced from the accept line ARDA of data interface 600. The multiplexer circuit 750-128, in accordance with the state of signal FARDA000, generates the two sets of signals MIFS2100, MIFS3100 and DMIFS2100, DMIFS3100 which are applied to the read address inputs of buffer 750-102.

It will be noted that section 750-115 also includes a double precision (FDPFSX) D type flip-flop 750-11517 which is set in response to clocking signal [CLKT020 to a binary ONE state via a first AND gate input in accordance with the state of the signal PTXDPFS100 applied to the AND gate via amplifier circuit 750-11518 from the DPFS line by SIU 100. The DPFS line when set indicates that two words of data are being sent from SIU 100. Switching occurs when SIU100 forces the signal PTXARDA100 applied thereto via an amplifier circuit 750-11519 from the ARDA line of interface 600 to a binary ONE. The ARDA line indicates that the read data requested by cache 750 is on the DFS lines from SIU100. The output of a FARDA flip-flop (not shown) which delays signal ARDA by one clock period is applied to a second hold AND gate input along with signal FDPFSX100. The FDPFSX flip-flop 750-11517 remains set for two clock periods. That is, the flip-flop 750-11517 is set in accordance with the number of SIU responses (DPFS signals). In the case of a read single command, the SIU generates two SIU responses, each response for bringing in a pair of words. In each case, this permits the writing of the two words into cache when signal RWRCACFLG100 is a binary ONE.

The binary ZERO output of flip-flop 750-11517 is inverted by a NAND/AND gate 750-11521 and delayed by a buffer delay circuit 750-11522 before it is applied to AND gate 750-11512. The same binary ZERO output without being inverted is delayed by a buffer delay circuit 750-11523 and applied to circuits which reset the states of bit positions of a transit buffer valid bit register which forms part of transit buffer 750-102.

It will also be noted that the double precision signal FDPF110 is combined in an AND gate 750-11524 with a write cache flag signal RWRTCACFLG100 from transit block buffer flag storage portion of buffer 750-102. The AND gate 750-1152 generates a memory write request signal MEMWRTREQ100 which is forwarded to section 750-9 for enabling memory data to be written into cache (i.e., controls address switch(s) selection).

As seen from FIG. 7a, the initiating request control circuits block 750-116 includes an active output port request flip-flop 750-11600. The flip-flop is a clocked D type flip-flop which includes two input AND/OR gating circuits. Flip-flop 750-11600 is set to a binary ONE state in response to clock signal [CLKT020 when block 750-114 forces a pair of signals ENABSETAOPR100 and SETAOPR100 to binary ONES. When set to a binary ONE, this, in turn, sets the AOPR line of interface 600, signalling the SIU100 of a data transfer request. The binary ZERO side of flip-flop 750-11600 is inverted by an inverter circuit 750-11602, delayed by a delay buffer circuit 750-11604 and applied to a hold AND gate. The flip-flop 750-11600 remains set until the clock time that signal FARA020 switches to a binary ZERO indicating that the SIU100 accepted the cache memory request.

The hold control block 750-117, as shown, includes an inhibit transit buffer hit FINHTBHIT flip-flop 750-11700, an AND gate 750-11702 and a plurality of AND/NAND gates 750-11704 through 750-11716. The flip-flop 750-11700 is set to a binary ONE state via a first input AND gate and a NAND gate 750-11701 in response to a T clock signal [CLKT020 when signals INHTBHIT100 and TBHIT100 are binary ONES. The NAND gate 750-11701 forces signal INHTBHIT100 to a binary ONE in the case of a cancel condition (i.e., signal [CANCELC012 is a binary ZERO).

The complement output side of flip-flop 750-11700 applies signal FINHTBHIT000 as one input to AND gate 750-11702. A directory busy signal DIRBUSY000 from block 750-526 is applied to the other input of AND gate 750-11702. When the directory is not performing a search (i.e., signal DIRBUSY000 is a binary ONE) and signal INHTBHIT100 is a binary ONE, AND gate 750-11702 forces signal INHTBACMP000 to a binary ONE. This, in turn, caues the gate 750-11704 to force signal TBHIT100 to a binary ONE when the AND gate 750-136 forces a transit block address compare signal TBACMP100 to a binary ONE. At the same time, gate 750-11704 forces signal TBHIT000 to a binary ZERO.

The AND/NAND gates 750-11708 through 750-11710 generate signals CPSTOP000 through CPSTOP003 which are forwarded to processor 700 for indicating a hold condition. The other AND/NAND gates 750-11714 through 750-11716 generate signals [HOLDDMEM000 through [HOLDDMEM003 to specify an internal hold condition for preventing the other sections of cache 750 from executing the command applied to the command lines by processor 700. Whenever there is a hold command condition (i.e., signal HOLDCMD000 is a binary ZERO), a miss condition (i.e., signal FRDMISS020 is a binary ZERO), a hold quad condition from block 750-916 (i.e., signal HOLDLDQUAD000 is a binary ZERO) or a transit block hit condition (i.e., signal TBHIT000 is a binary ZERO), the gates 750-11708 through 750-11710 force their respective output signals CPSTOP003 through CPSTOP000 to binary ZEROS and signals CPSTOP103 through CPSTOP100 to binary ONES. This, in turn, causes the processor 700 to halt operation.

Under similar conditions, in addition to a hold search condition (i.e., signal HOLDSEARCH000 is a binary ZERO) as indicated by AND gate 750-11712 forcing signal [EARLYHOLD000 to a binary ZERO or a hold cache condition (i.e., signal [HOLDCCU000 is a binary ZERO), the gates 750-11714 through 750-11716 force their respective output signals [HOLDDMEM000 through [HOLDDMEM003 to binary ZEROS and signals [HOLDDMEM100 through [HOLDDMEM103 to binary ONES.

Referring to the Figure, it is seen that the timing circuits of block 750-118 include a synchronous D type flip-flop 750-11800 with two AND/OR input circuits. The flip-flop 750-11800 receives a half T clocking signal [CLKHT100 via gate 750-11802 and inverter circuit 750-11804. A definer T clock signal DEFTCLK110 is applied to one of the data inputs via a pair of delay buffer circuits 750-11806 and 750-11808. Each buffer circuit provides a minimum delay of 5 nanoseconds.

Both the signals [CLKHT100 and DEFTCLK110 are generated by the common timing source. In response to these signals, the half T flip-flop 750-11800 switches to a binary ONE state upon the trailing edge of the DEFTCLK110 signal. It switches to a binary ZERO state upon the occurrence of the next [CLKHT100 signal (at the trailing edge).

The signals FHT100 and FHT000, in addition to signals FHT120, FHT010 and FHT020 derived from the binary ONE and binary ZERO output terminals of flip-flop 750-11800, are distributed to other circuits of section 750-1 as well as to other sections (i.e., 750-5, 750-9 and 750-114). The signals FHT120, FHT020 and FHT010 are distributed via another pair of delay buffer circuits 750-11810 and 750-11812 and a driver circuit 750-11814 respectively.

The T clock signals such as [CLKT020 and [CLKT022 generated by the common timing source are distributed in their "raw" form to the various flip-flops of registers. When there is a need to generate a 1/2 T clock signal, the 1/2 T clock signal [CLKHT020 is gated with the 1/2 T definer signal (FHT100) at the input of the flip-flop or register. The state of signal FHT100 is used to define the first and second halves of a T cycle. When signal FHT100 is a binary ONE, this defines a time interval corresponding to the first half of a T clock cycle. Conversely, when signal FHT100 is a binary ZERO, this defines a time interval corresponding to the second half of a T clock cycle.

For the purpose of the present invention, the data recovery circuits can be considered conventional in design and may, for example, take the form of the circuits described in the referenced patent applications. These circuits generate a data recovery signal for forwarding to processor 700 by "ANDING" the 1/2 T clock signal FHT000 with a signal indicating that data is being strobed into the processor's registers. This causes the data recovery signal to be generated only during the second half of a T clock cycle when such data is being strobed into the processor's registers.

In the case of sections 750-5 and 750-9, the signal FHT100 is used to control the switching of other timing and control flip-flops as explained herein.

DETAILED DESCRIPTION OF SECTION 750-3

FIG. 7b shows in greater detail specific ones of the blocks of section 750-3. Corresponding reference numbers have been used where possible.

Referring to FIG. 7b, it is seen that the decoder circuits of block 750-303 include a decoder circuit 750-30300 which is enabled for operation by signal ENBMEMLEV100 from the circuits of block 750-920. The signals from non-inverted output terminals of decoder circuit 750-30300 are applied to the input terminals of a first multiplexer circuit 750-30302. The signals at the inverted output terminals are applied to the input terminals of a second multiplexer circuit 750-30304. The multiplexer circuit 750-30302 is always enabled for operation while the multiplexer circuit 750-30304 is only enabled when signal ENBADR1100 is forced to a binary ONE by the circuits of block 750-920. It is assumed that the "0" positions of both multiplexer circuits will always be selected.

Predetermined combinations of the two sets of control signals [ZADR01100 through [ZADR71100 and signals [ZADR00100 through [ZADR70100 are applied to the control input terminals of each of the eight crossbar address selection switches 750-302a through 750-302h, as shown. It is seen that each crossbar switch includes a number of sections, each section includes three parts indicated by the heavy lines between sections. For simplicity, the number of sections of each switch are shown together. For simplicity, the control portion of each section is shown only once since it is the same for all the sections which are required to make up the switch.

As seen from the Figure, depending upon the states of the pairs of control signals [ZADR00100, [ZADR01100 through [ZADR70100, [ZADR71100, the signals from one of the three sources are applied to each set of W, X, Y and Z terminals simultaneously.

DETAILED DESCRIPTION OF SECTION 750-5

FIG. 7c shows in greater detail specific ones of the blocks of section 750-5 as explained previously. Corresponding reference numbers have been used where possible.

Referring to FIG. 7c, it is seen that the directory hit/miss control circuits of block 750-512 include an encoder network comprising a plurality of NAND gates 750-51200 through 750-51220 and a plurality of amplifier circuits 750-51224 through 750-51228. The NAND gate circuits are connected to encode the set of signals ZFE1100 through ZFE7100 from block 750-506 and the set of signals ZHT1100 through ZHT7100 from the blocks 750-546 through 750-552 into the 3-bit code for controlling the operation of switch 750-306.

The signal GSRCH100 is generated by the circuits of block 750-526. As explained herein, this signal is only forced to a binary ONE during the second half of a T clock cycle. Thus, an output from one of the NAND gates 750-51200 through 750-51208 is generated only during that interval. More specifically, the hit signal specified by the state of the full-empty bit causes one of the signals ZCDLEV1000 through ZCDLEV7000 to be forced to a binary ZERO state. This, in turn, conditions NAND gates 750-51216 through 750-51220 to generate the appropriate 3-bit code.

Signal ZCDICENAB100 also generated by the circuits of block 750-526 is forced to a binary ONE only during the first half of a T clock cycle. Thus, outputs from NAND gates 750-51210 through 750-51214 are generated only during that interval. That is, the instruction address level signals ZNICLEV0100 through ZNICLEV2100 from block 750-910 produce signals ICL0000 through ICL2000 which, in turn, produce signals ZCD0100 through ZCD2100. It will be noted that the signals ZCD0100 through ZCD2100 correspond to ZNICLEV0100 through ZNICLEV2100.

The signals RDDBLL0000 through RDDBLL2000 are used to define the second cycle of operation for a read double command. Accordingly, when any one of the signals RDDBLL0000 through RDDBLL2000 is in a binary ZERO state, this forces a corresponding one of the signals ZCD0100 through ZCD2100 to a binary ONE.

The signals ZCD0100 through ZCD2100 are applied to different inputs of corresponding ones of the amplifier driver circuits 750-51224 through 750-51228. These circuits apply the control signals [ZCD0100 through [ZCD2100 to the control terminals of switch 750-306.

A next block shown in greater detail in FIG. 7c is block 750-526. As mentioned previously, block 750-526 includes a number of directory control flip-flops. The control state flip-flop shown include the directory assignment (FDIRASN) control state flip-flop 750-52600 and a plurality of timing flip-flops of a register 750-52610.

The flip-flop 750-52600 is a clocked D type flip-flop which is set to binary ONE via first input AND gate in the case of a command request (i.e., signal REQCOMB0100 is a binary ONE) for a read type command (i.e., RDTYP100 is a binary ONE) when processor 700 requests data from memory and not cache 750 (i.e., signal BYPCAC110 is a binary ONE). In greater detail, in the absence of a hold condition (i.e., signal HOLD000 applied via an AND gate 750-52602 is a binary ONE), a go transfer (i.e., signal NOGO021 is a binary ONE), no cancel condition (i.e., signal CANCELC010 is a binary ONE) and processor 700 has signalled a request (i.e., signal DREQCAC111 is a binary ONE) an AND gate 750-52604 forces signal REQCOMB0100 to a binary ONE.

An AND gate 750-52606 forces the signal SETONBYP100 to a binary ONE in the case of read type when decoder circuit 750-528 forces signal RDTYP100 to a binary ONE when processor 700 forces the bypass cache signal BYPCAC110 to a binary ONE. The result is that the FDIRASN flip-flop 750-52600 switches to a binary ONE for specifying a directory assignment cycle of operation.

The flip-flop 750-52600 is also set to a binary ONE via a second input AND gate in the case of a command request (i.e., signal REQCOMB0100 is a binary ONE) when a miss condition is detected for the block requested to be read (i.e., signal SETONMISS100 is a binary ONE). The signal SETONMISS100 is forced to a binary ONE by an AND gate 750-52608 when signal RDTYP100 is a binary ONE and signal RAWHIT000 from block 750-512 is a binary ONE. The flip-flop 750-52600 is reset to a binary ZERO state upon the occurrence of clock signal [CLOCK112 generated from the common source in the absence of a set output signal from the two input AND gates.

A first flip-flop (FICENAB) of register 750-52610 is used to define the interval of time within a T clock cycle when instructions or operands are to be fetched from cache 750.

This flip-flop is switched to a binary ONE state via a first AND gate in response to a clock signal [CLOCKD120 when signal FHT100 generated by the timing circuits of block, 750-112 is a binary ONE. Clock signal [CLOCKD120 from the common timing source is applied via an AND gate 750-52612 and an inverter circuit 750-52612 and an inverter circuit 750-52514. The FICENAB flip-flop resets on the following clock signal when signal FHT-100 has been switched to a binary ZERO.

The second flip-flop of register 750-52610 is used to define an interval during which operands (not instructions) are being fetched from cache 750 as a consequence of a special condition caused by an IF1 command which did not specify the last word in an instruction block. The FRCIC flip-flop is switched to a binary ONE via a first input AND gate in response to clock signal [CLOCKD120 when signal FJAMZNICLEV000 is a binary ONE. The FRCIC flip-flop resets on the following clock pulse when signal FJAMZNICLEV000 has been switched to a binary ZERO.

As shown, the signal at the binary ZERO output terminal of the FICENAB flip-flop corresponds to the gate half T clock signal GATEHFTCHLK110 which is distributed to the circuits of block 750-920.

The signal FICENAB000 is combined with signal FRCIC000 and signal RDDBLZCDE000 within an AND gate 750-52616 to produce signal GSRCH100. The signal RDDBLZCDE000 is from decoder circuit. This gate forces signal GSRCH100 to a binary ONE during the second half of a T clock cycle when operands are being fetched (i.e., signal FICENAB000 is a binary ONE) except in the case of a read double command (i.e., signal RDDBLZCDE000 is a binary ONE).

The binary ZERO output of the FICENA flip-flop is combined with signal FRCIC000 within a NAND gate 750-52618. The NAND gate 750-52618 operates to force signal ZCDINCENAB100 to a binary ONE during the first half T interval when instructions are being fetched (i.e., signal FICENAB000 is a binary ZERO) or in the case of the type IF1 command described above (i.e., signal FRCIC000 is a binary ZERO).

The circuits of block 750-526 further include a NAND gate 750-52620 and a plurality of AND gates 750-52622 through 750-52628 connected, as shown. The circuits generate a first enable control signal DIRADDE100 for controlling the operation of decoder circuit 750-521. Additionally, they generate a second enable control signal FEDCODE100 for controlling the operation of a decoder circit 750-52000 of block 750-520.

In greater detail, during a directory assignment cycle (i.e., signal FDIRASN100 is a binary ONE) in the absence of a transfer no go condition (i.e., signal NOGO21 is a binary ONE), AND gate 750-52626 forces signal DIRNOGO100 to a binary ONE. When a signal FSKIPRR000 from the circuits of block 750-916 is a binary ONE, this causes the AND gate 750-52628 to force signal DIRADDE100 to a binary ONE which enables decoder circuit 750-521 for operation. When either signal DIRNOGO100 or KSKIPRR000 is forced to a binary ZERO, this causes AND gate 750-52628 to disable decoder circuit 750-521 by forcing signal DIRADDE100 to a binary ZERO.

Under the same conditions, the AND gate 750-52624 forces signal FEDCODE100 to a binary ONE which enables decoder circuit 750-52000 for operation. The AND gate 750-52630 causes an amplifier circuit 750-52632 to force signal FORCEBYP000 to a binary ONE when both signals FSKIPRR000 and FBYPCAC00 are binary ONES. The FORCEBYP000 is applied to the transit block flag section of block 750-102. The signal FBYPCAC000 is generated in a conventional manner in accordance with the signal applied to the line BYPCAC by processor 700. The signal is stored in a flip-flop, not shown, whose binary ZERO output corresponds to signal FBYPCAC000.

The circuits of block 750-520, as shown, include the decoder circuit 750-52000 and a pair of multiplexer circuits 750-52002 and 750-52004. It is assumed that normally the signals applied to the "0" input terminals of multiplexer circuits 750-52002 and 750-52004 are selected to be applied as outputs (i.e., the signal applied to the G input is a binary ZERO). Therefore, when the decoder circuit 750-520000 is enabled, the output signals FED0100 through FED7100 result in the generation of signals RWFE0100 through RWFE7100 in response to clock signal [CLOCK000.

The FIG. 7c also shown in greater detail register 750-504 as including a clocked four stage register 750-50400 and 750-50602. The register 750-50400 includes D type flip-flops, the first three of which are connected for storing round robin signals OLDRR0100 through OLDRR2100. The fourth flip-flop is connected to indicate the presence of an alternate hit condition having been detected by the circuits of block 750-562, not shown. That is, it is set to a binary ONE state which signal ALTHIT100 is a binary ONE.

It will be noted that the flip-flops of register 750-50400 are only enabled in response to clock signal [CLOCK112 when signal FDIRASN000 is a binary ONE indicative of no directory assignment cycle being performed (a hit condition).

In the case of a hit condition detected within the half of a block being referenced, signal ALTHIT000 is forced to a binary ZERO. This causes the first three flip-flops of register 750-50400 to be loaded via a first set of input AND gates with the round robin signals RR0100 through RR2100 from block 750-500. When there is a hit condition detected within the other half (alternate) of the block being referenced, the circuits of block 750-512 force signal ALTHIT100 to a binary ONE. This causes the three flip-flops to be loaded via a second set of input AND gates with the alternate level signals ALTHITLEV0100 through ALTHITLEV2100 generated by the circuits of block 750-512.

The binary ONE signals of register 750-50400 are applied as inputs to the amplifier driver circuits 750-50402 through 750-50406 for storage in the transit block buffer 750-102. The same signals are applied to the A operand input terminals of an adder circuit of block 750-508. The adder circuit adds or increments the signals OLDRR0100 through OLDRR2100 by one via the binary ONE applied to the C1 terminal of the adder circuit. The sum signals NXTRR0100 through NXTRR2100 generated at the F output terminals are written into the round robin section of control directory 750-500.

Lastly, the signals OLDRR0100 through OLDRR2100 are applied as inputs to another set of amplifier driver circuits 750-50408 through 750-50412 for storage in one of the instruction address registers 750-900 and 750-902 of FIG. 7e.

DETAILED DESCRIPTION OF SECTION 750-7

FIG. 7d shows in greater detail different ones of blocks of section 750-7. As seen from FIG. 7d, block 750-722 includes a plurality of series connected NAND gates 750-72230 through 750-72234. The NAND gates 750-72230 and 750-72231 are connected to receive instruction buffer valid and instruction control signals IBUF1V100, [ZRIB010 and IBUF2V100, [ZRIB100 from I buffers 750-715 and 750-717 and block 750-920. The IBUF1V100 and IBUF2V100 signals indicate the instruction buffer into which information is being loaded. That is, when signal IBUF1V100 is a binary ONE, that specifies that I buffer 750-715 is loaded. When signal IBUF2V100 is a binary ONE, that specifies that I buffer 750-717 is loaded with an instruction word.

The control signals [ZRIB010 and [ZRIB100 specify which instruction buffer valid bit is to be examined which corresponds to the instruction buffer being addressed. That is, when signal [ZRIB010 is a binary ONE, the IBUF1 valid bit is specified by the circuits of block 750-920. When signal [ZRIB100 is a binary ONE, that specifies the IBUF2 valid bit. When either signal IBUF1RDY000 or signal IBUF2RDY000 is forced to a binary ZERO, NAND gate 750-72232 forces signal TBIBUFRDY100 to a binary ONE indicative of a ready condition.

The circuits of block 750-920 force an enabling signal USETBRDY100 to a binary ONE following the switching of the appropriate I buffer valid bit. This causes the NAND gate 750-72233 to force the TBRDY000 signal to a binary ZERO. The result is that NAND gate 750-72234 forces the IBUFRDY100 to a binary ONE signalling the ready condition.

It will also be noted that NAND gate 750-72234 also forces the IBUFRDY100 signal to a binary ONE when an instruction fetch ready signal IFETCHRDY000 is forced to a binary ZERO by the circuits of block 750-920. Signal IFETCHRDY000 is a binary ONE except when the instructions are being pulled from a block in cache. Lastly, NAND gate 750-72234 forces IBUFRDY100 signal to a binary ONE when an instruction buffer compare signal IBUFCMPR000 is forced to a binary ONE by comparator circuit 750-11435.

As seen from FIG. 7d, the IBUF1 and IBUF2 sections 750-715 and 750-717 each include a plurality of 4×4 simultaneous dual read/write memories. Each memory is a 16-bit memory organized as 4 words of 4 bits each. Words may be independently read from any two locations at the same time as information is being written into any location.

The signals [WRTBUF0100 and [WRTBUF1100 are applied to the write address terminals of each memory while the signals [ZEXT0100 and [ZEXT1100 are applied to a different one of the sets of read address terminals together with the signal Vcc. The read address inputs are enabled by the Vcc signal applied to the G1 terminal. The signals [ZEXT0100 and [ZEXT1100 permit the read out of the contents of any one of the 4-bit locations to the 1AY output terminals.

The memories 750-71500 through 750-71503 of block 750-715 are loaded with the signals RDFS(XO)110 through RDFSP(X)010 and signal Vcc in response to T clock signal [CLKHT021 when IBUF1 strobe signal [IBUF1120 from clock 750-115 is forced to a binary ONE. Also, the contents of all the memory locations of each memory are cleared to ZEROS when the circuits of block 750-115 force IBUF1 reset signal RESIBUF1000 to a binary ONE. That is, as seen from FIG. 7d, the pairs of series connected NAND gates 750-71504 and 750-71505 and 750-71704 and 750-71705 force reset signals RESIBUF1000 and RESIBUF2000 to binary ONES. Also, an initialize signal INIT000 when forced to a binary ZERO by processor 700 also forces signals RESIBUF1000 and RESIBUF2000 to binary ONES.

In a similar fashion, the memories 750-71500 through 750-71503 of block 750-717 are loaded with signals RDFS(XO)110 through RDFSP(X)010 and signal Vcc in response to T clock signal [CLKHT021 when IBUF2 strobe signal is forced to a binary ONE. Since there are three additional groups of memories such as memories 750-71500 through 750-71503 included within each buffer to provide storage for four 36-bit instruction words, the (X) designation for input and output signals is used to indicate that such signals are similar except for the different values of (X). For example, XO has the values 00, 17, 18 and 35 while X8 has the values 08, 09, 26 and 27. Each byte location includes a valid bit location which is switched to a binary ONE whenever information is loaded into that location. However, the output signals from each valid bit byte location are connected in common so that the arrangement can be viewed such that each word location includes one valid bit location.

The memories may be considered conventional in design and for example, such memories may take the form of the circuits disclosed in U.S. Pat. No. 4,070,657.

DETAILED DESCRIPTION OF SECTION 750-9

FIG. 7e shows in greater detail specific ones of the blocks of section 750-9. Corresponding reference numbers have been used where possible.

Referring to FIG. 7e, it is seen that the block 750-920 includes a first group of circuits of block 750-92000 which generate the four sets of write control signals WRT00100 through WRT70100, WRT01100 through WRT71100, WRT02110 through WRT72100 and WRT03100 through WRT73100. As seen from FIG. 7e, these circuits include a pair of multiplexer circuits 750-92002 and 750-92004, a register 750-92006 and four octal decoder circuits 750-92008 through 750-92014, connected as shown.

The multiplexer circuit 750-92002 has signals RHITLEV0100 through RHITLEV2100 from block 750-512 applied to the set of "0" input terminals while signals RTVLEV0100 through RTVLEV2100 applied to the set of "1" input terminals. During the first half of a T cycle when signal FDFN2HT100 applied to the control terminal G0/G1 is a binary ZERO, the signals RHITLEV0100 and RHITLEV2100 are applied to the output terminals. They are clocked into the top three flip-flops of register 750-92006 in response to T clock signal [CLKHT02. This enables processor operands to be written into cache 750-300 during the second half of the T clock cycle. During the second half of a T cycle when signal FDFN2HT100 is forced to a binary ONE, the signals RTVLEV0100 through RTVLEV2100 are clocked into the register 750-92006 in response to the T clock signal [CLKHT02. This enables memory data to be written into cache 750-300 during the first half of the next cycle.

The second multiplexer circuit 750-92004 has signals ZONE0100 through ZONE3100 from switch 750-144 applied to the set of "0" input terminals while signal MEMWRTREQ100 from block 750-112 is applied to the set of "1" input terminals. When signal FDFN2HT100 is a binary ZERO, the signals ZONE0100 through ZONE3100 are applied to the output terminals. They are clocked into the bottom three flip-flops of register 750-9206 in response to T clock signal [CLKHT02. During the first half of a T clock cycle, NAND gate 750-92005 forces signal ENBWRT100 to a binary ONE which enables the previously loaded signals to be applied to the output terminals. This enables the processor zone bits to be used in specifying which operand bytes are to be updated when writing processor data into the specified level of cache. When signal FDFN2HT100 is forced to a binary ONE, the signal MEMWRTREQ100 is clocked into the register 750-92006. This causes all the zone bits to be forced to binary ONES for causing all of the bytes of each data word received from memory to be written into the specified level of cache during the first half of the next T clock cycle.

As seen from FIG. 7e, different ones of the signals RWRTLEV0100 through RWRTLEV2100 are applied to the enable input terminals of octal decoder circuits 750-92008 through 750-92014. The signals RWRTLEV0100 through RWRTLEV2100 are applied to the input terminals of each of the octal decoder circuits 750-92008 through 750-92014.

The block 750-920 includes a second group of circuits of block 750-92020. These circuits generate the half T clock signal applied to the circuits of block 750-92000, the enable memory level signal ENABMEMLEV100, and enable address signal ENADR1100 applied to the circuits of block 750-303. They also generate the sets of control signals [ZIC010, [ZIC110 and [RICA100, [RICB100 applied to the circuits of instruction address registers 750-900 and 750-902 in addition to control signals [RIRA100 and [RIRB100 applied to the registers 750-308 and 750-310.

The circuits of block 750-92020 include a pair of half definer flip-flops of a register 750-92022, a group of three control flip-flops of register 750-92024 and a clocked flip-flop 750-92026. The circuits also include a number of AND gates, NAND gates, AND/NAND gates and AND/OR gates 750-92030 through 750-92041.

The series connected AND/NAND gate 750-92030, AND/OR gate 750-92032 and AND gates 750-92034 and 750-92035 in response to a signal FLDQUAD100 from 750-916, a signal FWFIDESC010 from processor 700 and signals FACTVRIC000 and FACTVRIC100 from register 750-92024 generate control signals [ZIC000, [ZIC010 and [ZIC110. These signals are used to control the operation of ZIC switch 750-906 and the different sections of registers 750-900 and 750-902 (e.g. level valid bit storage and level bit storage) in addition to registers associated therewith.

The series connected AND gate 750-92036, the AND/NAND gate 750-92037 and NAND gates 750-92038 through 750-92041 operate to generate register strobe signals [RICA100 and [RICB100. These signals control the loading of registers 750-900 and 750-902. The AND gate 750-92036 forces signal VALRDIBUF100 to a binary ONE when a hit condition was detected in the case of a read command (i.e., signal FRDMISS000 is a binary ONE), the transfer was a go (i.e., signal NOG0020 is a binary ONE) and signal CMPDATA/ICLEV000 from the comparator circuit of block 750-912 is a binary ONE.

The signal FRDMISS000 is obtained from the binary ZERO output of the flip-flop, not shown, which as mentioned is set in accordance with the Boolean expression:

FRDMISS=(RDCMD.[HOLDDMEN.HITTOIC.[CANCELC).

The signals GOODFTCHA100 and GOODFTCHB100 generated by circuits, not shown, indicate whether the RICA register 750-900 or RICB register 750-902 is being used at that time and its contents are therefore incremented. For example, signal GOODFTCHA100 is generated in accordance with the following Boolean expression:

GOODFTCHA=INSTIF1.FLDQUAD.FACTVRIC.FDFN2HT+FDFN2HT. FLDQUAD.FACTRIC. Signal GOODFRCHB is generated in a similar fashion except for the reversal in states of signals FACTRIC and FACTVRIC.

It is seen that when signal EXECRDIBUF100 is forced to a binary ONE when processor 700 forces signal RDIBUF110 to a binary ONE, the NAND gate 750-92039 causes NAND gate 750-92041 to force signal [RICA100 to a binary ONE when signal GOODFTCHA100 is a binary ONE. The signal ENBSTRBA000 indicates when the RICA register 750-900 is being initially loaded. That is, when signal ENBSTRBA000 is forced to a binary ZERO, it causes NAND gate 750-92041 to force signal [RICA100 to a binary ONE. More specifically, signal ENBSTRBA is generated in accordance with the following Boolean expression:

ENBSTRBA=FLDQUAD.FACTVRIC.FNEWIF1.FDFN1HT+FDFN1HT.FACTVRIC.FJAMZNICLEV.FHOLDIF1+(INSTIF1+DCDLDQUAD).FACTRIC.FDFN2HT.[CANCLCMD+FDFN2HT.[ZIC.INH2HT.ENAB2HT. wherein ENAB2HT=ENABRIC1+ENABRIC2 and INH2HT=[CANCLCMD.FLASTINST. Under either set of conditions, signals [RICA100 and [RICB100 enable the strobing of their corresponding registers when they are either being initially loaded or following incrementing as when instructions are being fetched or pulled out from cache.

The NAND gate 750-92042, AND/NAND gate 750-92043 and NAND gates 750-92044 through 750-82049 are connected to generate register strobe signals [RIRA100 and [RIRB100 in a fashion similar to the generation of register strobe signals [RICA100 and [RICB100.

The NAND gate 750-92046 forces signal [RIRA100 to a binary ONE in the case of a new instruction fetch (i.e., signal NEWINST000 is a binary ZERO) or when the processor 700 takes an instruction from RIRA register 750-308 (i.e., signal TAKEINST000 is a binary ZERO). The NAND gate 750-92049 forces signal [RIRB100 in the case of a new operand fetch (i.e., signal NEWDATA000 is a binary ZERO) or when processor 700 takes a data word from RIRB register 750-310 (i.e., signal TAKEDATA000 is a binary ZERO).

The AND gate 750-92050 and AND/NAND gate 750-92051 generate signal ENBMEMLEV100 during the second half of a T clock cycle (i.e., signal FDFN2HT101 is a binary ONE) when the circuits of block 750-112 force memory write request signal MEMWRTREQ100 to a binary ONE. The NAND gate 750-92052 generates signal ENBADR1100 during the second half of a T clock cycle (i.e., signal FDFN1HT101 is a binary ZERO) or when the instruction counter is in use (i.e., signal USEIC000 is a binary ZERO).

As concerns the flip-flop registers, it is seen that the flip-flop of register 750-92026 is switched to a binary ONE state via a first AND gate when AND gate 750-92053 is conditioned to force signal INSTIF1100 to a binary ONE in response to an IF1 command being decoded by decoder circuit 750-922 (i.e., signal DCDIF1100 is a binary ONE) which does not require additional descriptors (i.e., signal FFPIMEIS020 from processor l700 is a binary ONE) and AND gate 750-92054 forces signal [CANCELCMD000 to a binary ONE in response to a no cancel condition (i.e., signal [CANCELC010 is a binary ONE) and a no hold condition (i.e., signal [HOLDDMEM001 is a binary ZERO).

The flip-flop register 750-92026 is reset to a binary ZERO via a second input AND gate which receives signals ENABNEWINST000 and NEWIF1FDBK100 from a pair of NAND gates 750-92042 and 750-92043 and AND gate 750-92055. The binary ONE output of the flip-flop register 750-92026 is applied to NAND gate 750-92056. NAND gate 750-92056, during the first half of a T clock cycle (i.e., signal FDFN1HT100 is a binary ONE), switches signal USEIC000 to a binary ZERO when signal FNEWIF1100 is switched to a binary ONE.

The second flip-flop register 750-92022 includes the pair of timing flip-flops which are both set to binary ONES in response to signal GATEHFTCLK100 from section 750-5 in response to 1/2 T clock signal [CLKHT021. The flip-flops of register 750-92022 are reset to binary ZEROS in response to the next 1/2 T clock signal [CLKHT021.

The flip-flops of register 750-92024, as mentioned previously, provide various state control signals. The first flip-flop (FRDIBUF) is switched to a binary ONE state when NAND gate 750-92060 forces signal SETRDIBUF100 to a binary ONE in response to read I buffer request from processor 700 (i.e., signal EXECRDIBUF000 is a binary ZERO) or an inhibit ready condition (i.e., signal FINHRDY010 is a binary ZERO) when AND gate 750-92061 forces signal ENABSETRDIBUF100 to a binary ONE. The signal ENABSETRDIBUFf100 is forced to a binary ONE in the case of a command which is not a load quad command (i.e., signal FLDQUAD000 is a binary ONE) or an instruction fetch 1 command (i.e., signal GOODIF1000 is a binary ONE). The FRDIBUF flip-flop is reset a clock period later in response to T clock signal [CLKT021 via a second input AND gate.

The second flip-flop (FACTVRIC) of register 750-92024 is set and reset in accordance with the Boolean expressions previously given via the NAND gates 750-92062 and 750-92064, the AND gate 750-92063 and AND/NAND gate 750-92065. The third flip-flop (FRDDATA) is set to a binary ONE state via a first input AND gate in response to signal SETRDIBUF100 when the compound is a load quad command (i.e., signal FLDQUAD100 is a binary ONE). The FRDDATA flip-flop is reset to a binary ZERO state a clock period later via a second input AND gate in response to the T clock signal [CLKT021.

The next group of circuits included within block 750-920 include the circuits of block 750-92070. As seen from FIG. 7e, these circuits include a first plurality of AND gates, AND/NAND gates and NAND gates 750-92071 through 750-92086, connected as shown. These gates generate control signals SETACURLEV100, [RICACNTL100 and RSTACURLEV2000 which control the setting and resetting of the current level and level valid vit positions of RICA register 750-900 in accordance with the states of signals SETALEV1VAL100, RSTALEV1VAL000 and SETLEV2VAL100. These signals are generated by another plurality of AND gates and NAND gates 750-92087 through 750-92095.

A second plurality of AND gates, AND/NAND gates and NAND gates 750-92100 through 750-92116, in a similar fashion, generates signals SETBCURLEV100, RSTBCURLEV200 and [RICBCNTL100 which set and reset the current level and valid bits for the RICB register 750-902 in accordance with signals SETBLEV1VAL100, RSTBLEV1VAL000 and SETBLEV2VAL100. These signals are generated by another plurality of AND gates and NAND gates 750-92120 through 750-92125.

A plurality of AND gates 750-92126 through 750-92129, in response to signals SETALEV1VAL100, SETBLEV1VAL100, SETALEV2VAL100 and SETBLEV1VAL100, generate control signals [RICALEV1100 through [RICBLEV2100 when signal [CANCELCMD000 is a binary ONE. These signals are applied to the control input terminals of the level bit storage sections of the RICA and RICB registers 750-900 and 750-902 for controlling the loading of hit level signals from section 750-512.

A further plurality of AND/NAND, AND/OR gates and NAND gates 750-92130 through 750-92137, in response to signals from the level vaid bit storage and level storage sections of registers 750-900 and 750-902, generate the use transit buffer ready signal USETBRDY100 and the control signals [ZRIB010 and [ZRIB100 which are applied to the circuits of block 750-114.

It is also seen that block 750-92070 includes a four D type flip-flop register 750-92140, the pair of ND gates 750-92141 and 750-92142, the pair of AND/NAND gates 750-92143 and 750-92144 and the pair of AND/OR gates 750-92145 and 750-92146, connected as shown. The flip-flops of register 750-92140 are loaded with the contents of bit positions 8 and 9 of the RICA and RICB registers 750-900 and 750-902 in response to T clock signal [CLKHT020 under the control of signals [RICA100 and [RICB100. That is, the top pair of register flip-flops are clocked when signal [RICA100 applied to terminal C1 is forced to a binary ONE while the bottom pair of register flip-flops are clocked when signal [RICB100 applied to terminal G2 is forced to a binary ONE. The signals [ZIC000 AND [ZIC100 applied to terminals G3 and G4 control independently the generation output signals from the top pair of flip-flops and bottom pair of flip-flops respectively at the corresponding sets of output terminals.

Pairs of binary ZERO output signals are combined within AND gates 750-92141 and 750-92142 to generate address signals ZEXT0100 and ZEXT1100, in addition to those signals required for the generation of control signal NEXTLEVVAL100 which is applied to the control input terminals of comparator circit 750-912.

A last group of circuits include a flip-flop register 750-92150 and a plurality of AND gates, an AND/NAND gate, NAND gates and AND/OR gates 750-92151 through 750-92156. These circuits are connected to generate signal IFETCHRDY000 which is applied to the circuits of section 750-114. The gates 750-92153 and 750-92154 are connected to generate timing signals DFN2HT101 and DFN2HT100 in response to signal FHT010 from block 750-112. These signals are forced to binary ONES during the second half of a T clock cycle of operation.

The flip-flop register 750-92150 is set to a binary ONE via a first input AND gate when AND gates 750-92151 and 750-92152 force signals SETINHRDY100 and CANCELINHRDY000 to binary ONES. It is reset to a binary ZERO via a second input AND gate when NAND gate 750-92155 forces signal RSINHRDY000 to a binary ZERO. The binary ZERO output of register 750-92150 is applied to AND/OR gate 750-92156. When signal FINHRDY000 is forced to a binary ZERO, it causes gate 750-92156 to force signal IFETCHRDY000 to a binary ONE state.

Additionally, FIG. 7e shows in greater detail the switch 750-910 and comparator circuits of blocks 750-912 and 750-914. The switch 750-910 is a crossbar switch which operates in the manner previously described. The W outputs select one of the two sets of signals applied to the A0 and A1 terminals in accordance with the state of signal [ZIC110. The X outputs select one of the two sets of signals applied to the A3 and A4 terminals in accordance with the state of signal [ZIC110. The Y and Z outputs select one of the four sets of signals applied to the A0-A4 terminals in accordance with the states of signals [ZIC110, [ZNICLEV100 and [ZIC110, ZCURLEV100.

The output signals ZNICLEV0100 through ZNICLEV2100 from the Y output terminals of circuit 750-910 are applied to the B input terminals of comparator circuit 750-912 for comparison with the signals RTBLEV0100 through RTBLEV2100 from section 750-102. The comparator circuit 750-912 is enabled when decoder circuit 750-922 has decoded an IF1 command (i.e., signal DECODEIF1010 is a binary ONE) and signal NEXTLEVVAL100 is a binary ONE. The comparison results in the generation of signals CMPDATA/ICLEV100 and CMPDATA/ICLEV000.

Other comparator circuits of blocks 750-912 and 750-914 operate in a similar manner to generate signals CMPCURLEV100 and CMPALTLEV100. In greater detail, another section of circuit 750-912 compares signals ZICLEV0100 through ZICLEV2100 with signals C7RR0100 through C7RR2100. When there is a true comparison, signal CMPCURLEV100 is forced to a binary ONE. This section is enabled via a NAND gate 750-91202 when either signal ZLEV1VAL000 or sigal ZLEV2VAL000 is a binary ZERO.

The comparator circuit 750-914 has two sections enabled by pairs of signals ZCURLEV100, ZLEV1VAL100 and ZCURLEV000, ZLEV2VAL100 as shown. The first section compares level 1 signals ZLEV10100 through ZlEV12100 with round robin signals C7RR0100 through C7RR2100. When there is a true comparison, the output signal at the A=B terminal is forced to a binary ZERO which causes NAND gate 750-91402 to force signal CMPALTLEV100 to a binary ONE.

In a similar fashion, the second section compares level 2 signals ZLEV20100 through ZLEV22100 with round robin signals C7RR0100 through C7RR2100. When there is a true comparison, the output signal is forced to a binary ZERO which causes NAND gate 750-91402 to force signal CMPALTLEV100 to a binary ONE.

DESCRIPTION OF OPERATION

With reference to FIGS. 1 through 7e, the operation of the preferred embodiment of the present invention will now be described.

As mentioned, the cycle arrangement of the preferred embodiment divides a T clock cycle into first and second halves. That is, when signal FHT100 is a binary ONE, this defines the first half of a T clock cycle. When signal FHT100 is a binary ZERO, this defines the second half of a T clock cycle.

During the first half of the T clock cycle, either instructions are fetched or memory data is written into cache store 750-300. In both cases, the level to be accessed is already established. That is, for instructions, the level is stored in either the RICA or RICB instruction address register at the time an IF1 or IF2 command received from processor 700 was executed. For memory data, the level is stored in one of the register locations of transit block buffer 750-102 as a result of the circuits of block 750-520 having detected a miss condition which caused cache 750 to fetch the requested data from memory. During the second half of a T clock cycle, either operand data is accessed from cache or processor data is written into cache in accordance with the results of a directory search.

To illustrate the operation of the preferred embodiment of the present invention, it will be assumed by way of example that processor 700 executes a first branch or transfer instruction causing the switching from a first sequence of instructions to a second sequence of instructions wherein the first instruction in that block/sequence is another branch instruction. In each case, it is assumed that the block or sequence of instructions switched to does not reside in cache store 750-300 and therefore must be fetched from main memory 800. Further, each transfer instruction is assumed to be a "GO".

For further information about the transfer instructions, reference may be made to the cited copending patent applications and in the publication "Series 60 (Level 66)/6000 MACRO Assembler Program (GMAP)" by Honeywell Information Systems Inc., Copyright 1977, Order Number DDOBB, Rev. 0.

As indicated herein, processor 700 carries out various operations during I, C and E cycles of operation in executing instructions. This results in the issuance of cache commands by processor 700 to cache unit 750 as described herein.

When processor 700 executes the first transfer or branch instruction, this results in the generation of an IF1 command followed by an IF2 command. It is assumed that prior to the transfer instruction, processor 700 had been fetching instructions from cache store 750-300 using the address and level information contained in RICB register 750-902.

The operation of cache unit 750 in executing the IF1 and IF2 commands now will be described. The IF1 command upon receipt by cache unit 750 is decoded by the decoder circuits 750-922. The decoder circuits 750-922 cause the circuits of block 750-920 to generate signals for loading the alternate instruction address register which is assumed to be RICA with signals corresponding to the incremented value of the address included within the IF1 command. That is, during the first T clock cycle, the address signals from switch 750-530 are incremented by one by circuit 750-912 and loaded into the address bit positions of RICA instruction address register 750-900 in response to 1/2 T clock signal [CLKHT100 when signal [RICA100 is a binary ONE. The signal [RICA100 is forced to a binary ONE by the circuits 750-920 when signal ENBSTRBA000 of FIG. 7e is forced to a binary ZERO during the first half of the first T clock cycle.

The IF1 command address is also applied as an input to the directory circuits of block 750-502 via ZDAD switch 750-530 for a search cycle of operation. Since the instruction block is not in cache, the circuits of block 750-512 generate the appropriate hit signals RAWHIT000, HITTOTB010 and HITTOIC010 indicative of a miss condition which are applied to sections 750-1, 750-5 and 750-9.

In the case of a miss condition, the circuits of block 750-526 of FIG. 7c switch the directory assignment flip-flop 750-52600 to a binary ONE in response to signal RAWHIT000 which is a binary ONE. The signal ALTHIT000 which is a binary ONE causes the round robin bit signals read out from directory 750-500 to be loaded into register 750-50400. The round robin signals are incremented by one so as to indicate the next level for replacement and are written back into directory 750-500 at the addressed location.

Also, the round robin signals from register 750-50400 are applied as signals TBRR0100-2100 to transit block buffer 750-102 for subsequent loading therein.

Also, the full-empty bits and high order bits of the IF1 command address (i.e., bits 10-23) are written into directories 750-500 and 750-502 at the location specified by the low order bits of the IF1 address (i.e., bits 24-33). On the next T clock, the FDIRASN flip-flop resets to a binary ZERO completing the directory assignment cycle.

It will be appreciated that due to the miss condition, the decoding of the IF1 command causes the level 1 valid bit and hit/miss positions of the RICA instruction address register 750-900 to be set to a binary ONE and binary ZERO (i.e., hit signal HITTOC7100 is a binary ZERO). Accordingly, the level signals loaded into the level 1 bit positions of RICA instruction address register 750-900 are ignored since the processor 700 will access the block of instructions fetched in response to the IF1 command from the IBUF1 section 750-715 and not from cache store 750-300 (i.e., signalled via the IBUFRDY and USETBRDY lines) as explained herein.

Prior to the switching of the FLDTBVALID flip-flop 750-11414 of FIG. 7a, before the directory assignment cycle, the write address signals FTBPTR0100-1100 from in pointer 750-106 are decoded which results in the writing of the IF1 command address into the next available location of transit block buffer 750-102. That is, on the T clock that completes the directory search, one of the transit block locations of buffer 750-102 is loaded with the IF1 address signals as a consequence of the miss condition. At that time, a corresponding location in the command queue 750-108 is loaded with the necessary control information required for transferring the IF1 command to memory.

Since the block of instructions requested are not in cache store 750-300, signal FRDMISS020 from the control miss flip-flop of section 750-9, not shown, is a binary ZERO while signal FRDMISS120 from the same flip-flop is a binary ONE. As seen from FIG. 7a, signal FRDMISS020 causes the AND/NAND gates 750-11708 through 750-11710 to force the CPU stop signals CPSTOP000-005 to binary ZEROS which is effective to stop or hold up processor operations on the subsequent T clock. At the same time, signals FRDMISS120 and FIF2SEARCH000 and NAND gate 750-11706 force signal HOLDSEARCH000 to a binary ZERO. This, in turn, causes the AND/NAND gates 750-11714 through 750-11716 to force the internal hold signals [HOLDDMEM000-003 to binary ZEROS. This holds up further operations being performed by cache sections 750-1, 750-5 and 750-9 until the IF1 directory assignment is completed and it is determined whether the IF1 command was due to a transfer type instruction and whether it was a GO or NO-GO condition.

During the period processor 700 is being held, the directory assignment is completed and the above determination is made. Where the IF1 command is due to a transfer instruction which is signalled by processor 700 as a NO-GO, the IF1 command stored in the transit block buffer 750-102 is cancelled as explained herein. In that case, the alternate RICB instruction register 750-902 becomes the current instruction address register and the address contained therein which is not incremented is used to access the next instruction from cache store 750-300 on the next 1/2 T clock.

Since it is assumed that the IF1 command is due to a transfer instruction which is a transfer GO indicating that the IF2 command is to be executed, the decoding of the IF2 command causes AND/OR gate 750-11402 to force signal SETIF2TIME100 to a binary ONE (i.e., signal NO-GO030 is a binary ONE) forces signal SETIF2SEARCH100 to a binary ONE. This switches the FIF2SEARCH flip-flop 750-11400 to a binary ONE in the absence of a cancel command signal being received from processor 700 (i.e., signal [CANCEL012 is a binary ONE).

The flip-flop 750-11400 forces signal FIF2SEARCH000 to a binary ZERO which causes NAND gate 750-11706 to force signal HOLDSEARCH000 to a binary ONE preventing further holding up of the internal operations of cache sections 750-1, 750-5 and 750-9.

At that time, since signal [HOLDMEM000 is a binary ONE on the T clock, the FLDTBVALID flip-flop 750-11414 is set to a binary ONE. That is, in the case of a miss condition, signal HITTOB010 causes the FLDTBVALID flip-flop 750-11414 of FIG. 7a to switch to a binary ONE on the T clock when the FDIRASN flip-flop is set. On the next T clock, this causes the contents of in pointers 750-106 and 750-108 to be incremented by one in preparation of the next command. The appropriate control flag bits are set and written into the buffer flag section of transit block buffer 750-102.

As seen from FIG. 7a, the decoding of the IF1 command causes AND gate 750-11420 to switch the FIF1ASSIGN flip-flop 750-11418 to a binary ONE state in the case of a miss condition (i.e., signal SETLDTBVALID100 is a binary ONE). Additionally, the AND gate 750-11426 switches the FRDQUAD flip-flop 750-11410 to a binary ONE as a consequence of the miss.

The write flag and read quad flag bit positions of flag storage 750-10238 to be forced to binary ONES as a result of signals FORCEBYP000 and FRDQUAD100 being binary ONES. The signal FORCEBYP000 is generated by the AND circuit 750-52630 and is normally a binary ONE. The signal FRDQUAD100 is generated by the FRDQUAD flip-flop when the FLDTBVALID flip-flop 750-11414 switches to a binary ONE.

On the T clock of the directory assignment cycle, the IF1 command is read out from transit block buffer 750-102, in response to out pointer address signals from command queue 750-107, into the RDTS register 750-119 via the ZTBC position of ZDTS switch 750-118. The level signals TBRR0100-2100 are loaded into the addressed transit buffer location on the 1/2 T clock. The IF1 command is transferred to SIU100 on the DTS lines via switch 750-102 of FIG. 4. The appropriate memory identifier signals are loaded into the RMITS register 750-124 and steering signals into the steering register (not shown). These signals are applied to the MITS and SDTS lines, respectively. For further information regarding the generation and use of steering signals, reference may be made to U.S. Pat. No. 4,006,466.

As mentioned, each transit block location has both an I fetch 1 and I fetch 2 flag flip-flop associated therewith. As seen from FIG. 7a, during the I fetch 1 assignment cycle, the I fetch 1 flag flip-flop corresponding to the location specified by write pointer signals FTBPTR0000-1000 decoded by decoder circuit 750-10601 is set to a binary ONE state. At the same time, the other three I fetch 1 flag flip-flops are reset to binary ZEROS. That is, when the in pointer is pointing to location 0, then signal IN0100 is a binary ONE while signals IN1100-3100 are binary ZEROS. This causes the FIF10 flip-flop to be set to a binary ONE and the FIF11-13 flip-flops to be reset to binary ZEROS. The multiplexer circuit 750-1255 is conditioned by signal FIF1ASSIGN100 to select as outputs the signals IN0100-3100 applied to its position 1 terminals which results in the I fetch 1 flip-flop associated with that location being set to a binary ONE. When signal FIF1ASSIGN100 switches to a binary ZERO, the multiplexer circuit 750-10255 is conditioned to select as outputs the signals HOLD0100-3100 applied to its position 0 terminals. This is effective to hold the I fetch 1 flag flip-flop in a binary ONE state.

The execution of the IF2 command by cache unit 750 is similar to the IF1 command. The IF2 address is applied as an input to the directory circuits of block 750-502 via ZDAD switch 750-530 for a search cycle of operation.

Again, prior to the directory assignment cycle (i.e., when the FLDTBVALID flip-flop 750-11414 is a binary ZERO), the write address signals FTBPTR0100-1100 from in pointer 750-106 are decoded and cause the IF2 command to be written into the next available location of transit block buffer 750-102.

On the next T clock, the FLDTBVALID flip-flop 750-11414 is again set to a binary ONE together with the switching of FDIRASN flip-flop. On the next T clock, the contents of the in pointers 750-106 and 750-108 are incremented by one in preparation of the next command. Also, the appropriate control flag bits are set and written into the buffer flag section of buffer 750-102 in the manner previously described.

During the directory assignment cycle, the level signals from round robin register 750-50400 are incremented by one and written back into directory 750-500. Also, the round robin signals are applied as signals TBRR0100-2100 to transit block buffer 750-102 for subsequent loading therein. Further, the full-empty bits and high order bits of the IF2 address (i.e., bits 10-23) are written into directories 750-500 and 750-502, respectively at the locations specified by the low order bits of IF2 address (i.e., bits 24-30).

Again, due to the miss condition, the decoding of the IF2 command causes the level 2 valid bit and hit/miss positions of the RICA instruction address register 750-900 to be set to a binary ONE and a binary ZERO, respectively (i.e., hit signal HITTOC7100 is a binary ZERO). Again, the level signals loaded into the level 2 bit positions of RICA instruction address register 750-900 are ignored since processor 700 pulls the block of instructions fetched in response to the IF2 command from the IBUF2 section 750-717 and not from cache store 750-300.

As seen from FIG. 7a, during the I fetch 2 assignment cycle defined by the FIF2ASSIGN flip-flop 750-11410 switching to a binary ONE in response to an IF2 command (signal FIF2SEARCH010 is a binary ZERO), the I fetch 2 flag flip-flop corresponding to the location specified by the write pointer signals FTBPTR0000-1000 decoded by circuit 750-10601 is set to a binary ONE state while the other three I fetch 2 flag flip-flops are reset to binary ZEROS. That is, signal FIF2ASSIGN100 conditions multiplexer circuit 750-10256 to select as outputs position 1 signals IN0100-3100. The I fetch 2 flip-flop associated with the location containing the IF2 command is switched to a binary ONE while the remaining three flip-flops are reset to binary ZEROS.

On the T clock of the directory assignment cycle, the IF2 command is read out from the transit block buffer 750-102, in response to out pointer address signals from command queue 750-107 into the RDTS register 750-119 via the ZTBC position of ZDTS switch 750-118. The level signals TBRR0100-2100 are loaded into the addressed transit buffer location on the 1/2 T clock. The IF2 command is transferred to SIU100 in the same manner as described above.

It will be appreciated that each of the instructions loaded into RDFS register 750-702 on a T clock received in response to the IF1 command are loaded into one of locations of the IBUF1 section 750-715 specified by signals [WRTBUF0110-1110 on the subsequent 1/2 T clock.

In greater detail, when the SIU100 begins the transfer of information, the circuits of block 750-115 force memory write request signal to a binary ONE. That is, the SIU100 forces the DPFS line to a binary ONE indicating that the first two words are being transferred to cache unit 750. It also forces the ARDA line to a binary ONE to indicate that the requested information is on the DFS lines. The presence of these two signals together with the write cache flag signal read out from buffer 750-102 being set to a binary ONE cause the circuits of block 750-115 to force the memory write request signal MEMWRTREQ100 to a binary ONE. At this time, the SIU applies signals to the MIFS lines, bits 2 and 3 corresponding to signals MIFS2100 and MIFS3100 condition the transit block buffer 750-102 to read out the address and level signals contained in the location storing the IF1 command for writing the pair of words transferred to RDFSB register 750-712 on the T clock into cache store 750-300. These signals also address the corresponding location within the flag section which read out the write cache flag signal associated with the IF1 command.

Additionally, referring to FIG. 7a, it is seen that bits 2 and 3 corresponding to signals DMIF2100 and DMIF3100 condition multiplexer circuit 750-10260 to select the pair of I fetch 1 and I fetch 2 flag flip-flops associated with the transit block location storing the IF1 command. The outputs of these flip-flops provide flag signals ZIF1FLG100 and ZIF2FLG100 which are applied to the circuits of block 750-115.

As seen from FIG. 7a, the signals ZIF1FLG100 and ZIF2FLG100 when both binary ONES cause the NAND gates 750-11508 and 750-11509 to force signals [IBUF1100 and [IBUF2100 to binary ONES during the first half of a T clock cycle (i.e., when signal FHT100 is a binary ONE). These signals enable IBUF1 and IBUF2 sections 750-715 and 750-717. The write address signals [WRTBUF1100 and [WRTBUF0100 are generated in response to signals FARDA010 and RMIFS1100. These signals address one of the locations of the IBUF1 and IBUF2 sections for writing therein on the 1/2 T clock the instruction word contained in RDFS register 750-720. This operation is repeated for each instruction word. This causes the circuits of block 750-114 to force the IBUFRDY line to a binary ONE.

In greater detail, referring to FIG. 7e, it is seen that the states of the signals ZLEV1LOC000 and ZLEV1LOC100 generated by AND/NAND gate 750-92130 in accordance with the setting of the hit/miss bit position of RICA instruction address register 750-900 conditions the AND/OR gate 750-92137 to force signal USETBRDY100 to a binary ONE and the same signals condition gate 750-92156 to force signal IFETCHRDY000 to a binary ONE indicating that instructions are not to be pulled from cache store 750-300. The signal ENABTBRDY100 is forced to a binary ONE by multiplexer circuits 750-10255 and 750-10256 following the completion of an instruction fetch assignment cycle at which time signals FIF1ASSIGN100 and FIF2ASSIGN100 are both binary ZEROS. The result is that the NAND gate 750-72233 is conditioned to force signal TBRDY000 to a binary ZERO causing NAND gate 750-72234 to force signal IBUFRDY100 to a binary ONE. At that time, processor 700 by forcing the RDIFUF line to a binary ONE loads the instruction word applied to the ZDI lines into its RBIR register for processing. At that time, processor 700 is released or restarted (i.e., signal FRDMISS020 is a binary ONE).

When all four instructions of the block have been written into cache store 750-300, the execution of the IF1 command is completed. At that time, the valid bit indicator associated with the transit block location containing the IF1 command is reset to a binary ZERO.

It will be appreciated that where the first instruction is transferred to processor 700 immediately, it is possible to have both an IF1 and IF2 command undergoing execution when processor 700 executes another branch or transfer instruction. In this case, there are instructions outstanding for both the IF1 and IF2 commands.

In the manner previously described, processor 700 processes the branch instruction which results in the generation of another pair of IF1 and IF2 commands with the same assumption that the transfer is a go. These commands are processed by cache unit 750 in the same manner as described above.

As soon as the circuits of section 750-1 of FIG. 7a are signalled that the IF1 command is in response to a transfer instruction which is a go, and the FLDTBVALID flip-flop 750-11414 is set to a binary ONE at which time the new IF1 command has been loaded into one of the transit block buffer locations, the circuits of block 750-114 set the I fetch 1 flag flip-flop associated with that location to a binary ONE. At the same time, the remaining three I fetch 1 flip-flops including the one set in response to the previous IF1 command are automatically reset to binary ZEROS.

In a similar fashion, upon the subsequent loading of the new IF2 command into transit buffer 750-102, the circuits of block 750-114 set the I fetch 2 flag flip-flop associated with that location to a binary ONE. At the same time, the remaining three I fetch 2 flip-flops including the one set in response to the previous IF2 command are automatically reset to binary ZEROS. This completes the I fetch assignment cycle at which time signals FIF1ASSIGN100 and FIF2ASSIGN100 are binary ZEROS as a result of FLDTBVALID flip-flop 750-11414 switching to a binary ZERO.

Accordingly, the instruction words received in response to the previous IF1 and IF2 commands are not written into the IBUF1 and IBUF2 sections 750-715 and 750-717 as a consequence of resetting the I fetch 1 and I fetch 2 flag flip-flops. That is, when MIFS bits 2 and 3 received from SIU100 condition multiplexer circuit 750-10260 to select the outputs of the I fetch 1 and I fetch 2 flag flip-flops associated with transit buffer location storing the IF1 command, the signals ZIF1FLG100 and ZIF2FLG100 are forced to binary ZEROS. This inhibits the enabling of IBUF1 and IBUF2 sections. Hence, the instruction words fetched for that IF1 command are not written into the instruction buffer, but are only written into cache store 750-300 in the manner discussed above.

The same thing is true for the instruction words fetched in response to the IF2 command. Accordingly, only the instruction words fetched in response to the new IF1 and IF2 commands are written into the instruction buffer as well as cache store 750-300.

Because of the arrangement of the present invention, the cache unit 750 can continue issuing IF1/IF2 commands without having to wait for the instruction words of a previously issued IF1/IF2 command to be received. In effect, the arrangement permits the previous IF1/IF2 command to be cancelled. Thus, a succession of such commands can be issued without regard to having the instructions fetched in response to such commands affecting the processing of these blocks of instructions fetched in response to a new IF1/IF2 command.

Additionally, in accordance with the preferred embodiment of the present invention, in the case of a branch instruction command which is a NO-GO, this inhibits the FIF2SEARCH flip-flop from being switched to a binary ONE. Additionally, signal NOGO030 also inhibits the FLDTBVALID flip-flop 750-11414 and FDIRASN flip-flop 750-52600 from being switched to binary ONES. Further, the FRDMISS control flip-flop included in section 750-9 is switched to a binary ZERO state via signal NOGO030. Accordingly, the valid bit indicator bit associated with the transit block buffer location into which the IF1/IF2 command is to be loaded remains a binary ZERO. Also, the IF1/IF2 flag flip-flops associated therewith remain binary ZEROS. Thus, the command is ignored and processor 700 is permitted to continue processing operations. Also, the cache unit 750 is able to process a new IF1/IF2 command immediately (i.e., no internal hold signals generated).

Thus, the arrangement of the present invention facilitates the processing of IF1/IF2 commands by enabling overlap in the processing of such commands and the issuing of new commands with a minimum amount of interruption of processor operations.

It will be obvious to those skilled in the art that many changes may be made to the preferred embodiment without departing from the teachings of the present invention. Such changes may be made in the number of indicators or groups of indicators. For example, where it is desirable to fetch a single group of instructions, only a single group of instruction fetch flag indicator storage circuits would be required. Also, other changes will occur to those skilled in the art.

While in accordance with the provisions and statute, there has been illustrated and described the best form of the invention known, certain changes may be made to the system described without departing from the spirit of the invention as set forth in the appended claims and that in some cases, certain features of the invention may be used to advantage without a corresponding use of other features. 

What is claimed is:
 1. A cache unit for use with a data processing unit for providing fast access to data and instruction words fetched from a main store coupled to said cache unit in response to commands received from said data processing unit, said cache unit comprising:an instruction buffer coupled to said main store and having a plurality of addressable locations for storing sequences of said instruction words; bistable instruction indicator means for storing a group of indicators corresponding in number to at least the maximum number of a predetermined type of read commands which can be in process at any one time; and, control means coupled to said indicator means, to said processing unit and to said instruction buffer, said control means including first means operative in response to each predetermined type of read command received from said processing unit to generate signals for switching one of said indicators of said group associated with said received predetermined type of read command from a first state to a second state simultaneous with switching the remaining indicators of said group to said first state and said control means further including second means coupled to said indicator means and to said instruction buffer, said second means being conditioned by said indicators for enabling said instruction buffer for writing therein only those instruction words transferred to said cache unit in response to a last one of said predetermined type of read command received from said processing unit as defined by the states of said group of indicators.
 2. The cache unit of claim 1 wherein said first and second states correspond to a binary ZERO and binary ONE respectively.
 3. The cache unit of claim 1 wherein said instruction buffer further includes a number of sections, each section having said plurality of addressable locations for storing a predetermined sequence of instruction words; and,wherein said bistable instruction indicator means includes a plurality of sets of bistable elements corresponding in number to said maximum number of read commands, said first means being operative in response to each predetermined type of read command coded to specify fetching instruction words of one of said predetermined sequences to generate signals for switching a first one of said bistable elements of one of said sets associated with said read command from said first state to said second state simultaneous with said switching of corresponding first ones of said bistable elements of the remaining sets to said first state and said second means being conditioned by said one of said sets of bistable elements to enable the designated one of said number of sections for writing therein only those instruction words transferred to said cache unit in response to said predetermined type of read command specified by the states of said first ones of said bistable elements.
 4. The cache unit of claim 3 wherein said number of said sections is two, one of said sections for storing a first sequence of instruction words and the other of said sections for storing a second sequence of instruction words and wherein the number of bistable elements in said set is two, one bistable element of said set being switched from said first state to said second state in response to a predetermined type of read command coded to specify fetching instruction words of said first sequence and the other bistable element of said set being switched from said first state to said second state in response to a predetermined type of read command coded to specify fetching instruction words of said second sequence.
 5. The cache unit of claim 4 wherein said predetermined type of read command coded to specify fetching said instruction words of said first sequence corresponds to an instruction fetch 1 command coded to designate the transfer of instruction words of a first half of a block of instruction words into said one of said sections and wherein said predetermined type of read command coded to specify fetching said instruction words of said second sequence corresponds to an instruction fetch 2 command coded to designate the transfer of the second half of said block into said other section.
 6. The cache unit of claim 3 wherein said first means includes:addressing means individually coupled to each of said plurality of sets of bistable elements, said addressing means generating signals for enabling the switching of one of said bistable elements of a next available set to said second state in response to said predetermined type of read command and for switching said corresponding ones of said bistable elements of said remaining sets to said first state.
 7. The cache unit of claim 6 wherein each of said bistable elements has an input terminal and an output terminal and wherein said bistable instruction indicator means further includes:first multiplexer circuit means having a first set of input terminals, each being coupled to said addressing means and to said output terminal of a first bistable element of a different one of said sets, a second set of input terminals coupled to said addressing means, a plurality of output terminals, each coupled to said input terminal of said first bistable element of said different one of said sets; and, second multiplexer circuit means having a first set of input terminals, each coupled to said addressing means and to said output terminal of a second bistable element of said different one of said sets, a second set of input terminals, each coupled to said addressing means and a plurality of output terminals, each coupled to said input terminal of said second bistable element of said different one of said sets; and, wherein said control means further includes: enabling means coupled to said first and second multiplexer circuit means, said enabling means being operative in response to said predetermined type of read command specifying the fetching of instruction words of said first or second sequences to generate first or second control signals for enabling said first and second multiplexer circuit means respectively to apply to said output terminals, said signals from said addressing means for switching one of said first bistable elements or said second bistable elements of said next available set designated by said address signals to said second state simultaneous with resetting said first or second bistable elements of said remaining sets to said first state.
 8. The cache unit of claim 7 wherein said first multiplexer circuit means further includes a first input terminal connected to a voltage corresponding to said second state, a second input terminal connected to a reference signal corresponding to said first state and an output terminal and wherein said second multiplexer circuit means further includes a first input terminal connected to said first multiplexer circuit output terminal, a second input terminal connected to said reference signal and an output terminal, said first and second multiplexer circuit means being operative in the absence of said first and second control signals to apply said voltage from said first multiplexer circuit means input terminal to said output terminal of said second multiplexer circuit means for signalling the completion of an instruction fetch assignment cycle of operation in response to a sequence of said predetermined type of read commands specifying fetching of said first and second sequences of instruction words enabling subsequent transfer of said instruction words to said processing unit.
 9. The cache unit of claim 8 wherein said second means includes:output multiplexer circuit means including:a first number of input terminals, each coupled to said output terminal of a different one of said first bistable elements; a second number of input terminals, each coupled to said output terminal of a corresponding different one of said second bistable elements; a number of control terminals coupled to receive a set of identifier signals identifying the predetermined type of read command in response to which instruction words are being transferred to said cache unit by said main store; and, a pair of output terminals, each coupled to one of said instruction buffer sections, said output multiplexer circuit means in response to said set of identifier signals being conditioned to apply signals from one of said first bistable elements and said corresponding one of said second bistable elements designated by said set of identifier signals to said pair of output terminals indicating which one of said instruction buffer sections is to be enabled for operation for writing said instruction words therein.
 10. The cache unit of claim 6 wherein said cache unit further includes:a transit block buffer including a number of locations for storing said maximum number of read commands, said transit block buffer being coupled to said addressing means and responsive to said signals for enabling storage of said predetermined type of read command into a next available one of said locations associated with said next available set of bistable elements.
 11. The cache unit of claim 10 wherein said cache unit further includes:a buffer store organized into a plurality of levels, each level containing a number of blocks of said word locations; and, wherein said transit block buffer further includes:a first section having said maximum number of locations, each location for storing a block address and level signals specifying the level and block of word locations within said buffer store into which a block of words fetched from said main store are to be written; and, a second section having said maximum number of locations, each location having a number of bit positions for storing a corresponding number of control flag indications, said first and second sections being coupled to said addressing means and conditioned by said addressing means to write the block address and level designated by said predetermined type of read command into said next available location of said first section and to switch a predetermined one of said number of bit positions of said next available location of said second section to a predetermined state for indicating when instruction words fetched in response to said predetermined type of read command are to be written into said block and said level of said buffer store independently of the states of said first and second bistable elements of said set associated therewith.
 12. The cache unit of claim 11 wherein said second means further include:output selection circuit means for generating control signals for specifying which one of said instruction buffer sections is to be enabled for operation, said output selection circuit means being coupled to said sets of said first and second bistable elements and to said instruction buffer, said output selection circuit means being operative in response to a set of memory identifier signals received from said main store to select the first and second bistable elements of the set designated by said set of memory identifier signals for generating said control signals.
 13. The cache unit of claim 12 wherein said control means further includes:bistable means for signalling the completion of loading said predetermined type of read command into said transit block buffer, said bistable means being coupled to said first and second bistable elements of said sets, said bistable means being switched from a first to a second state in response to each predetermined type of read command in response to a signal indicating that the instructions requested by said command are not stored in said buffer store, said bistable means when in said second state enabling said transit block buffer to complete the storage of said command in said next available location and the switching of one of said first and second bistable elements designated by said predetermined type of read command of said next available set to said second state.
 14. The cache unit of claim 13 wherein said bistable means includes input means coupled to said processing unit for receiving a no go signal indicating that the instruction transfer specified by said predetermined type of read command is not made, said bistable means being inhibited by said no go signal from switching to said second state in response to said predetermined type of read command and said bistable means when in said first state preventing completion of the storage of said command and the switching of said one of said first and second bistable elements of said next available set to said second state.
 15. A cache unit for use with a data processing unit for providing fast access to data and instructions fetched from a main store coupled to said cache unit in response to commands received from said data processing unit, said cache unit comprising:an instruction buffer coupled to said main store and having a plurality of addressable locations for storing sequences of instruction words; a transit block buffer comprising:a number of locations for storing a maximum number of different types of read commands in process at any one time; and, a plurality of bistable means corresponding in number of said number of transit block locations, each of said bistable means being associated with a different one of said transit block locations; control means coupled to said processing unit, to said transit block buffer and to said instruction buffer, said control means including first means and second means, said first means coupled to said transit block buffer and being operative in response to a predetermined type of read command from said processing unit to generate signals for storing said predetermined type of read command into a next available one of said transit buffer locations and for switching a corresponding one of said plurality of bistable elements from a first state to a second state simultaneous with switching the remaining ones of said plurality of bistable elements to said first state, and each bistable means when in said second state conditioning said second means to enable said instruction buffer for writing said instruction words fetched in response to a last one of said predetermined type of read command received from said processing unit designated by the states of said plurality of bistable elements.
 16. The cache unit of claim 15 wherein said instruction buffer further includes a number of sections, each section having said plurality of addressable locations for storing a predetermined one of said sequences of instruction words; and,wherein said plurality of bistable means includes a plurality of sets of bistable elements corresponding in number of transit block buffer locations, said first means being operative in response to each predetermined type of read command coded to specify fetching instruction words of said predetermined one of said sequences to generate signals for switching a first bistable element of one of said sets of bistable elements from said first state to said second state simultaneous with said switching of first bistable elements of the remaining sets of said first state and said second means being conditioned by said one of said sets to enable the designated one of said number of sections for writing therein only those instruction words transferred to said cache unit in response to said last one of said predetermined type of read command.
 17. The cache unit of claim 16 wherein said number of said sections is two, a first section for storing a first sequence of instruction words and a second section for storing a second sequence of instruction words and wherein the number of bistable elements within said set is two, said first bistable element being switched from said first state to said second state in response to a predetermined type of read command coded to specify fetching instruction words of said first sequence and a second bistable element being switched from said first state to said second state in response to a predetermined type of read command coded to specify fetching instruction words of said second sequence.
 18. The cache unit of claim 17 wherein said predetermined type of read command coded to specify fetching said instruction words of said first sequence corresponds to an instruction fetch 1 command coded to designate the transfer of instruction words of the first half of a block of instruction words into said first section and wherein said predetermined type of read command coded to specify fetching said instruction words of said second sequence corresponds to an instruction fetch 2 command coded to designate the transfer of the second half of said block into said second section.
 19. The cache unit of claim 16 wherein said first means of said control means includes:addressing means individually coupled to each of said plurality of sets of bistable elements, said addressing means generating signals for enabling the switching of one of said bistable elements of a next available set to said second state in response to said predetermined type of read command and for switching said corresponding ones of said bistable elements of said remaining sets to said first state.
 20. The cache unit of claim 19 wherein each of said bistable elements has an input terminal and an output terminal and wherein said control means further includes:first multiplexer circuit means having a first set of input terminals, each being coupled to said addressing means and to said output terminals of said first bistable element of a different one of said sets, a second set of input terminals coupled to said addressing means, a plurality of output terminals coupled to said input terminals of said first bistable element of said different one of said sets; second multiplexer circuit means having a first set of input terminals, each coupled to said addressing means and to said output terminal of said second bistable element of said different one of said sets, a second set of input terminals, each coupled to said addressing means and a plurality of output terminals, each coupled to said input terminal of said second bistable element of said different one of said sets; and, enabling means coupled to said first and second multiplexer circuit means, said enabling means being operative in response to said predetermined type of read command specifying the fetching of instruction words of said first or second sequence to generate a first or a second control signal for enabling said first multiplexer circuit means to apply to said output terminals, said signals from said addressing means for switching one of said first bistable elements or one of said second bistable elements of said next available set designated by said address signals to said second state simultaneous with resetting said first or second bistable elements of said remaining sets to said first state.
 21. The cache unit of claim 20 wherein said second means includes:output multiplexer circuit means including:a first number of input terminals, each coupled to said output terminal of a different one of said first bistable elements; a second number of input terminals, each coupled to said output terminal of a corresponding different one of said second bistable elements; a number of control terminals coupled to receive a set of identifier signals identifying the predetermined type of read command in response to which instruction words are being transferred to said cache unit by said main store; and, a pair of output terminals, each coupled to one of said instruction buffer sections, said output multiplexer circuit means in response to said set of identifier signals being conditioned to apply signals from one of said first bistable elements and said corresponding one of said second bistable elements designated by said set of identifier signals to said pair of output terminals indicating which one of said instruction buffer sections is to be enabled for operation for writing said instruction words therein.
 22. The cache unit of claim 19 wherein said cache unit further includes:a buffer store organized into a plurality of levels, each level containing a number of blocks of said word locations; and, wherein said transit block buffer further includes:a first section having said number of locations, each location for storing a block address and level signals specifying the level and block of word locations within said buffer store into which a block of words fetched from said main store are to be written; and, a second section having said number of locations, each location having a number of bit positions for storing a corresponding number of control flag indications, said first and second sections being coupled to said addressing means and conditioned by said addressing means to write the block address and level designated by said predetermined type of read command into said next available location of said first section and to switch a predetermined one of said number of bit positions of said next available location of said second section to a predetermined state for indicating when instruction words fetched in response to said predetermined type of read command are to be written into said block and said level of said buffer store independently of the states of said first and second bistable elements of said set associated therewith.
 23. The cache unit of claim 22 wherein said second means further includes:output selection circuit means for generating control signals for specifying which one of said instruction buffer sections is to be enabled for operation, said output selection circuit means being coupled to said sets of said first and second bistable elements and to said instruction buffer, said output selection circuit means being operative in response to a set of memory identifier signals received from said main store to select the first and second bistable elements of the set designated by said set of memory identifier signals for generating said control signals.
 24. The cache unit of claim 23 wherein said control means further includes:bistable means for signalling the completion of loading said predetermined type of read command into one of said locations of said transit block buffer, said bistable means being coupled to said first and second bistable elements of said sets, said bistable means being switched from a first to a second state in response to each predetermined type of read command in response to a signal indicating that the instructions requested by said command are not stored in said buffer store, said bistable means when in said second state enabling said transit block buffer to complete the storage of said command in said next available location and the switching of one of said first and second bistable elements designated by said predetermined type of read command of said next available set to said second state.
 25. The cache unit of claim 24 wherein said bistable means includes input means coupled to said processing unit for receiving a no go signal, said bistable means being inhibited by said no go signal from switching to said second state in response to said predetermined type of read command and said bistable means when in said first state preventing completion of the storage of said command and the switching of said one of said first and second bistable elements of said next available set to said second state.
 26. A cache system for use with a data processing unit for providing fast access to data and instructions fetched from a main store coupled to said cache unit in response to commands received from said data processing unit, said cache system comprising:first and second instruction buffers coupled to said main store, each having a plurality of addressable locations for storing first or second sequences of instruction words; a transit block buffer having a number of locations for storing the maximum number of different types of read commands which can be in process at any one time; a plurality of pairs of bistable elements corresponding in number to said number of transit block locations each pair being associated with a different one of said transit block locations; control means coupled to said processing unit, to said plurality of pairs of bistable elements, to said transit block buffer and to said instruction buffers, said control means including first and second means coupled to said plurality of pairs of bistable elements, said first means being operative in response to each instruction fetch read command from said processing unit specifying the fetching of a first or second sequence of instruction words to generate signals for storing said each read command into a next available one of said transit buffer locations and for switching a first one of said bistable elements of a corresponding one of said pairs designated by said instruction fetch command from a first state to a second state simultaneous with switching said first ones of said bistable elements of the remaining ones of said pairs to said first state, and said second means enabling one of said instruction buffers designated by the states of said bistable elements of said corresponding one of said pairs for writing therein said instruction words of the sequence.
 27. The cache system of claim 26 wherein a first bistable element of said pair is switched from said first state to said second state in response to a first instruction fetch read command coded to specify fetching instruction words of said first sequence and a second bistable element of said pair being switched from said first state to said second state in response to a second instruction fetch read command coded to specify fetching instruction words of said second sequence.
 28. The cache system of claim 26 wherein said first means includes:addressing means individually coupled to said first and second bistable elements of each of said pairs, said addressing means generating signals for enabling the switching of one of said bistable elements of a next available pair to said second state in response to said instruction fetch read command and for switching said corresponding ones of said bistable elements of said remaining ones of said pairs to said first state.
 29. The cache system of claim 28 wherein each of said bistable elements has an input terminal and an output terminal and wherein said second means includes:first multiplexer circuit means having a first set of input terminals, each being coupled to said addressing means and to said output terminals of said first bistable element of a different one of said pairs, a second set of input terminals coupled to said addressing means and a plurality of output terminals coupled to said input terminals of said first bistable element of said different one of said pairs; second multiplexer circuit means having a first set of input terminals, each coupled to said addressing means and to said output terminal of said second bistable element of said different one of said pairs, a second set of input terminals, each coupled to said addressing means and a plurality of output terminals, each coupled to said input terminal of said second bistable element of said different one of said pairs; and, enabling means coupled to said first and second multiplexer circuit means, said enabling means being operative in response to said instruction fetch read command specifying the fetching of instruction words of said first or second sequence to generate a first or second control signal for enabling said first multiplexer circuit means to apply to said output terminals, said signals from said addressing means for switching one of said first bistable elements or one of said second bistable elements of said next available pair designated by said address signals to said second state simultaneous with resetting said first or second bistable elements of said remaining pairs to said first state.
 30. The cache system of claim 29 wherein said second means further includes:output multiplexer circuit means including:a first number of input terminals, each coupled to said output terminal of a different one of said first bistable elements; a second number of input terminals, each coupled to said output terminal of a corresponding different one of said second bistable elements; a number of control terminals coupled to receive a set of identifier signals identifying the instruction fetch read command in response to which instruction words are being transferred to said cache system by said main store; and, a pair of output terminals, each coupled to one of said instruction buffers, said output multiplexer circuit means in response to said set of identifier signals being conditioned to apply signals from one of said first bistable elements and said corresponding one of said second bistable elements designated by said set of identifier signals to said pair of output terminals indicating which one of said instruction buffers is to be enabled for operation for writing said instruction words therein.
 31. The cache system of claim 28 wherein said cache unit further includes:a cache store organized into a plurality of levels, each level containing a number of blocks of said word locations; and, wherein said transit block buffer further includes:a first section having said number of locations, each location for storing a block address and level signals specifying the level and block of word locations within said cache store into which a block of words fetched from said main store are to be written; and, a second section having said number of locations, each location having a number of bit positions for storing a corresponding number of control flag indications, said first and second sections being coupled to said addressing means and conditioned by said addressing means to write the block address and level designated by said instruction fetch read command into said next available location of said first section and to switch a predetermined one of said number of bit positions of said next available location of said second section to a predetermined state for indicating when instruction words fetched in response to said predetermined type of read command are to be written into said block and said level of said buffer store independently of the states of said first and second bistable elements of said pair associated therewith.
 32. The cache system of claim 31 wherein said control means further includes:output selection circuit means for generating control signals for specifying which one of said instruction buffer sections is to be enabled for operation, said output selection circuit means being coupled to said sets of said first and second bistable elements and to said instruction buffer, said output selection circuit means being operative in response to a set of memory identifier signals received from said main store to select the first and second bistable elements of the set designated by said set of memory identifier signals for generating said control signals.
 33. The cache system of claim 32 wherein said control means further includes:bistable means for signalling the completion of loading said instruction fetch read command into one of said locations of said transit block buffer, said bistable means being coupled to said first and second bistable elements of said pairs, said bistable means being switched from a first to a second state in response to each instruction fetch read command in response to a signal indicating that the instructions requested by said command are not stored in said cache store, said bistable means when in said second state completing the storage of said command in said next available location and the switching of said first or second bistable elements designated by said instruction fetch command of said next available pair to said second state.
 34. The cache system of claim 33 wherein said bistable means includes input means coupled to said processing unit for receiving a no go signal, said bistable means being inhibited by said no go signal from switching to said second state in response to said predetermined type of read command and said bistable means when in said first state preventing completion of the storage of said command and the switching of said one of said first and second bistable elements of said next available pair to said second state. 